diff --git a/NEWS b/NEWS
index 5acaf162608b..3aefec858702 100644
--- a/NEWS
+++ b/NEWS
@@ -15,6 +15,8 @@ PHP                                                                        NEWS
   . Uploading multiple files with the same field name is now possible by
     passing an array of CURLFile or CURLStringFile objects to
     CURLOPT_POSTFIELDS. (Sjoerd Langkemper)
+  . Added CURLOPT_OPENSOCKETFUNCTION to allow or refuse each connection libcurl
+    is about to make. (Xavier Leune)
 
 - FPM:
   . Use recommended TYPE, HELP order for OpenMetrics metadata.
diff --git a/UPGRADING b/UPGRADING
index 3fc692ac9ac0..465e20661a5c 100644
--- a/UPGRADING
+++ b/UPGRADING
@@ -76,6 +76,14 @@ PHP 8.7 UPGRADE NOTES
     array of CURLFile or CURLStringFile objects to CURLOPT_POSTFIELDS.
   . Support for upstream options CURLOPT_ECH, CURLOPT_HAPROXY_CLIENT_IP, and
     CURLOPT_SERVER_RESPONSE_TIMEOUT_MS.
+  . Added CURLOPT_OPENSOCKETFUNCTION. Its callback has the signature
+    function(CurlHandle $handle, string $address, int $port): bool and is
+    invoked after DNS resolution and before connect(), for each socket libcurl
+    is about to create. $address is the IP address, or the path of a UNIX
+    socket, whose $port is 0. Returning false refuses the connection, and
+    libcurl tries the next address, if any. A connection reused from a pool is
+    not vetted again; CURLOPT_PREREQFUNCTION runs before every request.
+    RFC: https://wiki.php.net/rfc/curl_opensocketfunction
 
 - Intl:
   . Added IntlRelativeDateTimeFormatter to format relative date and time offsets
@@ -172,6 +180,7 @@ PHP 8.7 UPGRADE NOTES
     CURLOPT_SERVER_RESPONSE_TIMEOUT_MS.
   . CURLOPT_MAIL_RCPT_ALLOWFAILS is the correctly spelled alternative to
     CURLOPT_MAIL_RCPT_ALLLOWFAILS.
+  . CURLOPT_OPENSOCKETFUNCTION.
 
 ========================================
 11. Changes to INI File Handling
diff --git a/ext/curl/curl.stub.php b/ext/curl/curl.stub.php
index bac8694da631..13873a98c4d1 100644
--- a/ext/curl/curl.stub.php
+++ b/ext/curl/curl.stub.php
@@ -257,6 +257,11 @@
  * @cvalue CURLOPT_NOSIGNAL
  */
 const CURLOPT_NOSIGNAL = UNKNOWN;
+/**
+ * @var int
+ * @cvalue CURLOPT_OPENSOCKETFUNCTION
+ */
+const CURLOPT_OPENSOCKETFUNCTION = UNKNOWN;
 /**
  * @var int
  * @cvalue CURLOPT_PORT
diff --git a/ext/curl/curl_arginfo.h b/ext/curl/curl_arginfo.h
index bf47f682b1b2..d68ce84542e9 100644
--- a/ext/curl/curl_arginfo.h
+++ b/ext/curl/curl_arginfo.h
@@ -1,5 +1,5 @@
 /* This is a generated file, edit curl.stub.php instead.
- * Stub hash: 74051be5006c0e0d66dced8f60f2fa4128ecad48 */
+ * Stub hash: 9703f6327501da9ceb4a65550df9ad498c4d4069 */
 
 #include "zend_attributes.h"
 #include "zend_constants.h"
@@ -283,6 +283,7 @@ static void register_curl_symbols(int module_number)
 	REGISTER_LONG_CONSTANT("CURLOPT_NOBODY", CURLOPT_NOBODY, CONST_PERSISTENT);
 	REGISTER_LONG_CONSTANT("CURLOPT_NOPROGRESS", CURLOPT_NOPROGRESS, CONST_PERSISTENT);
 	REGISTER_LONG_CONSTANT("CURLOPT_NOSIGNAL", CURLOPT_NOSIGNAL, CONST_PERSISTENT);
+	REGISTER_LONG_CONSTANT("CURLOPT_OPENSOCKETFUNCTION", CURLOPT_OPENSOCKETFUNCTION, CONST_PERSISTENT);
 	REGISTER_LONG_CONSTANT("CURLOPT_PORT", CURLOPT_PORT, CONST_PERSISTENT);
 	REGISTER_LONG_CONSTANT("CURLOPT_POST", CURLOPT_POST, CONST_PERSISTENT);
 	REGISTER_LONG_CONSTANT("CURLOPT_POSTFIELDS", CURLOPT_POSTFIELDS, CONST_PERSISTENT);
diff --git a/ext/curl/curl_private.h b/ext/curl/curl_private.h
index f8550dc3f740..9fec873dbe02 100644
--- a/ext/curl/curl_private.h
+++ b/ext/curl/curl_private.h
@@ -83,6 +83,7 @@ typedef struct {
 	zend_fcall_info_cache xferinfo;
 	zend_fcall_info_cache fnmatch;
 	zend_fcall_info_cache debug;
+	zend_fcall_info_cache opensocket;
 #if LIBCURL_VERSION_NUM >= 0x075000 /* Available since 7.80.0 */
 	zend_fcall_info_cache prereq;
 #endif
diff --git a/ext/curl/interface.c b/ext/curl/interface.c
index b94949b784c2..861f48460626 100644
--- a/ext/curl/interface.c
+++ b/ext/curl/interface.c
@@ -26,7 +26,15 @@
 
 #ifdef PHP_WIN32
 #include <winsock2.h>
+#include <ws2tcpip.h>
 #include <sys/types.h>
+#else
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+# ifdef AF_UNIX
+#  include <sys/un.h>
+# endif
 #endif
 
 #include <curl/curl.h>
@@ -36,6 +44,8 @@
 #include "ext/standard/info.h"
 #include "ext/standard/file.h"
 #include "ext/standard/url.h"
+#include "ext/standard/php_net.h"
+#include "php_network.h"
 #include "curl_private.h"
 
 #ifdef __GNUC__
@@ -499,6 +509,10 @@ static HashTable *curl_get_gc(zend_object *object, zval **table, int *n)
 		zend_get_gc_buffer_add_fcc(gc_buffer, &curl->handlers.debug);
 	}
 
+	if (ZEND_FCC_INITIALIZED(curl->handlers.opensocket)) {
+		zend_get_gc_buffer_add_fcc(gc_buffer, &curl->handlers.opensocket);
+	}
+
 #if LIBCURL_VERSION_NUM >= 0x075000 /* Available since 7.80.0 */
 	if (ZEND_FCC_INITIALIZED(curl->handlers.prereq)) {
 		zend_get_gc_buffer_add_fcc(gc_buffer, &curl->handlers.prereq);
@@ -837,6 +851,133 @@ static int curl_ssh_hostkeyfunction(void *clientp, int keytype, const char *key,
 }
 #endif
 
+/* {{{ curl_opensocket_unix_path
+   The path of a UNIX domain socket, as libcurl built it. A Linux abstract socket
+   name starts with a NUL byte, which is kept: it is part of the name. */
+#if defined(AF_UNIX) && !defined(PHP_WIN32)
+static zend_string *curl_opensocket_unix_path(const struct curl_sockaddr *address)
+{
+	const struct sockaddr_un *sa_un = (const struct sockaddr_un *) &address->addr;
+	size_t offset = offsetof(struct sockaddr_un, sun_path);
+	size_t len = address->addrlen > offset ? address->addrlen - offset : 0;
+
+	if (len > sizeof(sa_un->sun_path)) {
+		len = sizeof(sa_un->sun_path);
+	}
+	if (len > 0 && sa_un->sun_path[0] != '\0') {
+		/* A pathname: libcurl may count the terminating NUL in addrlen. */
+		len = zend_strnlen(sa_un->sun_path, len);
+	}
+
+	return zend_string_init(sa_un->sun_path, len, 0);
+}
+#endif
+/* }}} */
+
+/* {{{ curl_opensocketfunction
+   The PHP callback only allows or refuses the connection: no descriptor is ever
+   exposed to userland. */
+static curl_socket_t curl_opensocketfunction(void *clientp, curlsocktype purpose, struct curl_sockaddr *address)
+{
+	php_curl *ch = (php_curl *) clientp;
+
+	/* Unreachable in normal operation: libcurl's own socket creation is restored
+	 * whenever no callback is installed, so the trampoline is only registered
+	 * while the FCC is initialized. Refuse rather than connect, so that an
+	 * unexpected state can never silently bypass the filter. */
+	if (UNEXPECTED(!ZEND_FCC_INITIALIZED(ch->handlers.opensocket))) {
+		return CURL_SOCKET_BAD;
+	}
+
+#if PHP_CURL_DEBUG
+	fprintf(stderr, "curl_opensocketfunction() called\n");
+	fprintf(stderr, "purpose = %d, family = %d, socktype = %d, protocol = %d\n", purpose, address->family, address->socktype, address->protocol);
+#endif
+
+	/* Whatever cannot be described to the callback is refused rather than
+	 * connected: a policy must never be bypassed by an endpoint it was not
+	 * shown. CURLSOCKTYPE_IPCXN is the only purpose libcurl currently uses; a
+	 * future one may well not be a destination address at all. */
+	if (UNEXPECTED(purpose != CURLSOCKTYPE_IPCXN)) {
+		return CURL_SOCKET_BAD;
+	}
+
+	zval args[3];
+	zval retval;
+	curl_socket_t rval = CURL_SOCKET_BAD;
+
+	switch (address->family) {
+		case AF_INET:
+			ZVAL_LONG(&args[2], ntohs(((const struct sockaddr_in *) &address->addr)->sin_port));
+			break;
+#ifdef AF_INET6
+		case AF_INET6:
+			ZVAL_LONG(&args[2], ntohs(((const struct sockaddr_in6 *) &address->addr)->sin6_port));
+			break;
+#endif
+#if defined(AF_UNIX) && !defined(PHP_WIN32)
+		case AF_UNIX:
+			/* CURLOPT_UNIX_SOCKET_PATH or CURLOPT_ABSTRACT_UNIX_SOCKET: a path
+			 * has no port, reported as 0 like CURLINFO_PRIMARY_PORT does. */
+			ZVAL_LONG(&args[2], 0);
+			break;
+#endif
+		default:
+			return CURL_SOCKET_BAD;
+	}
+
+#if defined(AF_UNIX) && !defined(PHP_WIN32)
+	if (address->family == AF_UNIX) {
+		ZVAL_STR(&args[1], curl_opensocket_unix_path(address));
+	} else
+#endif
+	{
+		zend_string *ip = php_inet_ntop(&address->addr);
+		if (UNEXPECTED(ip == NULL)) {
+			return CURL_SOCKET_BAD;
+		}
+		ZVAL_STR(&args[1], ip);
+	}
+
+	GC_ADDREF(&ch->std);
+	ZVAL_OBJ(&args[0], &ch->std);
+
+	php_curl_call_callback(ch, &ch->handlers.opensocket, &retval, /* argc */ 3, args);
+
+	/* retval is undefined when the callback threw, and must not be trusted when
+	 * an exception was thrown after it was set, e.g. by a destructor: either
+	 * way the connection is refused and the exception propagates out of
+	 * curl_exec(). */
+	if (!Z_ISUNDEF(retval)) {
+		_php_curl_verify_handlers(ch, /* reporterror */ true);
+		if (UNEXPECTED(EG(exception))) {
+			/* Refused. */
+		} else if (EXPECTED(Z_TYPE(retval) == IS_TRUE || Z_TYPE(retval) == IS_FALSE)) {
+			if (Z_TYPE(retval) == IS_TRUE) {
+				rval = socket(address->family, address->socktype, address->protocol);
+				/* Where socket() supports SOCK_NONBLOCK, libcurl only makes a
+				 * socket non-blocking if it came from this callback, which it
+				 * checks after the call: if the callback removed the option,
+				 * the socket would stay blocking and ignore the connect
+				 * timeout. Doing it here is harmless otherwise. */
+				if (rval != CURL_SOCKET_BAD && php_set_sock_blocking(rval, false) != SUCCESS) {
+					closesocket(rval);
+					rval = CURL_SOCKET_BAD;
+				}
+			}
+		} else {
+			zend_type_error("The CURLOPT_OPENSOCKETFUNCTION callback must return a bool");
+		}
+		zval_ptr_dtor(&retval);
+	}
+
+	zval_ptr_dtor(&args[0]);
+	zval_ptr_dtor(&args[1]);
+
+	return rval;
+}
+/* }}} */
+
 /* {{{ curl_read */
 static size_t curl_read(char *data, size_t size, size_t nmemb, void *ctx)
 {
@@ -1154,6 +1295,7 @@ void init_curl_handle(php_curl *ch)
 	ch->handlers.progress = empty_fcall_info_cache;
 	ch->handlers.xferinfo = empty_fcall_info_cache;
 	ch->handlers.fnmatch = empty_fcall_info_cache;
+	ch->handlers.opensocket = empty_fcall_info_cache;
 	ch->handlers.debug = empty_fcall_info_cache;
 #if LIBCURL_VERSION_NUM >= 0x075000 /* Available since 7.80.0 */
 	ch->handlers.prereq = empty_fcall_info_cache;
@@ -1332,6 +1474,7 @@ void _php_setup_easy_copy_handlers(php_curl *ch, php_curl *source)
 	php_curl_copy_fcc_with_option(ch, CURLOPT_XFERINFODATA, &ch->handlers.xferinfo, &source->handlers.xferinfo);
 	php_curl_copy_fcc_with_option(ch, CURLOPT_FNMATCH_DATA, &ch->handlers.fnmatch, &source->handlers.fnmatch);
 	php_curl_copy_fcc_with_option(ch, CURLOPT_DEBUGDATA, &ch->handlers.debug, &source->handlers.debug);
+	php_curl_copy_fcc_with_option(ch, CURLOPT_OPENSOCKETDATA, &ch->handlers.opensocket, &source->handlers.opensocket);
 #if LIBCURL_VERSION_NUM >= 0x075000 /* Available since 7.80.0 */
 	php_curl_copy_fcc_with_option(ch, CURLOPT_PREREQDATA, &ch->handlers.prereq, &source->handlers.prereq);
 #endif
@@ -1727,6 +1870,27 @@ static zend_result _php_curl_setopt(php_curl *ch, zend_long option, zval *zvalue
 		HANDLE_CURL_OPTION_CALLABLE(ch, CURLOPT_FNMATCH_, handlers.fnmatch, curl_fnmatch);
 		HANDLE_CURL_OPTION_CALLABLE(ch, CURLOPT_DEBUG, handlers.debug, curl_debug);
 
+		case CURLOPT_OPENSOCKETFUNCTION: {
+			bool installed = php_curl_set_callable_handler(&ch->handlers.opensocket, zvalue, is_array_config, "CURLOPT_OPENSOCKETFUNCTION");
+			if (!installed || !ZEND_FCC_INITIALIZED(ch->handlers.opensocket)) {
+				/* Restore libcurl's own socket creation, so that the trampoline
+				 * is registered if and only if the FCC is initialized. A
+				 * rejected callable also releases the previous handler, so
+				 * leaving the option armed would point CURLOPT_OPENSOCKETDATA
+				 * at a handle whose FCC is gone, and curl_copy_handle() would
+				 * then carry that pointer into the copy. */
+				curl_easy_setopt(ch->cp, CURLOPT_OPENSOCKETFUNCTION, NULL);
+				curl_easy_setopt(ch->cp, CURLOPT_OPENSOCKETDATA, NULL);
+				if (!installed) {
+					return FAILURE;
+				}
+				break;
+			}
+			curl_easy_setopt(ch->cp, CURLOPT_OPENSOCKETFUNCTION, curl_opensocketfunction);
+			curl_easy_setopt(ch->cp, CURLOPT_OPENSOCKETDATA, ch);
+			break;
+		}
+
 #if LIBCURL_VERSION_NUM >= 0x075000 /* Available since 7.80.0 */
 		HANDLE_CURL_OPTION_CALLABLE(ch, CURLOPT_PREREQ, handlers.prereq, curl_prereqfunction);
 #endif
@@ -2925,6 +3089,9 @@ static void curl_free_obj(zend_object *object)
 	if (ZEND_FCC_INITIALIZED(ch->handlers.debug)) {
 		zend_fcc_dtor(&ch->handlers.debug);
 	}
+	if (ZEND_FCC_INITIALIZED(ch->handlers.opensocket)) {
+		zend_fcc_dtor(&ch->handlers.opensocket);
+	}
 #if LIBCURL_VERSION_NUM >= 0x075000 /* Available since 7.80.0 */
 	if (ZEND_FCC_INITIALIZED(ch->handlers.prereq)) {
 		zend_fcc_dtor(&ch->handlers.prereq);
@@ -3017,6 +3184,10 @@ static void _php_curl_reset_handlers(php_curl *ch)
 		zend_fcc_dtor(&ch->handlers.debug);
 	}
 
+	if (ZEND_FCC_INITIALIZED(ch->handlers.opensocket)) {
+		zend_fcc_dtor(&ch->handlers.opensocket);
+	}
+
 #if LIBCURL_VERSION_NUM >= 0x075000 /* Available since 7.80.0 */
 	if (ZEND_FCC_INITIALIZED(ch->handlers.prereq)) {
 		zend_fcc_dtor(&ch->handlers.prereq);
diff --git a/ext/curl/tests/curl_opensocketfunction_abstract_unix_socket.phpt b/ext/curl/tests/curl_opensocketfunction_abstract_unix_socket.phpt
new file mode 100644
index 000000000000..f04a9e950c09
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_abstract_unix_socket.phpt
@@ -0,0 +1,30 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION reports an abstract UNIX socket name with its leading NUL byte
+--EXTENSIONS--
+curl
+--SKIPIF--
+<?php
+if (PHP_OS_FAMILY !== 'Linux') die('skip abstract UNIX sockets are Linux only');
+if (!defined('CURLOPT_ABSTRACT_UNIX_SOCKET')) die('skip CURLOPT_ABSTRACT_UNIX_SOCKET not available');
+?>
+--FILE--
+<?php
+/* The leading NUL byte is part of an abstract name: keeping it tells a policy
+ * that the name is not a filesystem path. */
+$name = 'php-curl-' . getmypid();
+
+$ch = curl_init('http://localhost/');
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+curl_setopt($ch, CURLOPT_ABSTRACT_UNIX_SOCKET, $name);
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle, string $address, int $port) use ($name): bool {
+    var_dump($address === "\0" . $name, $port);
+    return false;
+});
+var_dump(curl_exec($ch));
+var_dump(curl_errno($ch) === CURLE_COULDNT_CONNECT);
+?>
+--EXPECT--
+bool(true)
+int(0)
+bool(false)
+bool(true)
diff --git a/ext/curl/tests/curl_opensocketfunction_address_family.phpt b/ext/curl/tests/curl_opensocketfunction_address_family.phpt
new file mode 100644
index 000000000000..bcdddc1ca216
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_address_family.phpt
@@ -0,0 +1,62 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION reports the address and port of IPv4 and IPv6 endpoints
+--EXTENSIONS--
+curl
+--SKIPIF--
+<?php
+if (!(curl_version()['features'] & CURL_VERSION_IPV6)) die('skip libcurl built without IPv6');
+/* The IPv6 probes below rely on libcurl attempting a literal ::1 address, which
+ * it will not do on a host without a usable IPv6 loopback. */
+$s = @stream_socket_server('tcp://[::1]:0', $errno, $errstr);
+if ($s === false) die('skip no IPv6 loopback available');
+fclose($s);
+?>
+--FILE--
+<?php
+/* No listener is needed: the callback runs before connect(). Proxies from the
+ * environment are ignored, or the callback would see the proxy instead. */
+function probe(string $url): void {
+    $ch = curl_init();
+    curl_setopt($ch, CURLOPT_URL, $url);
+    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+    curl_setopt($ch, CURLOPT_PROXY, '');
+    curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle, string $address, int $port): bool {
+        var_dump($address, $port);
+        return false;
+    });
+    curl_exec($ch);
+}
+
+echo "IPv4 literal\n";
+probe('http://127.0.0.1:1/');
+
+echo "\nIPv6 literal\n";
+probe('http://[::1]:1/');
+
+/* An IPv4-mapped IPv6 address reaches IPv4 from an AF_INET6 socket. Both forms
+ * normalise to the same string. */
+echo "\nIPv4-mapped IPv6 address\n";
+probe('http://[::ffff:127.0.0.1]:1/');
+
+echo "\nIPv4-mapped IPv6 address, hexadecimal form\n";
+probe('http://[::ffff:7f00:1]:1/');
+
+echo "Done";
+?>
+--EXPECT--
+IPv4 literal
+string(9) "127.0.0.1"
+int(1)
+
+IPv6 literal
+string(3) "::1"
+int(1)
+
+IPv4-mapped IPv6 address
+string(16) "::ffff:127.0.0.1"
+int(1)
+
+IPv4-mapped IPv6 address, hexadecimal form
+string(16) "::ffff:127.0.0.1"
+int(1)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_clone.phpt b/ext/curl/tests/curl_opensocketfunction_clone.phpt
new file mode 100644
index 000000000000..47cfcbe051c6
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_clone.phpt
@@ -0,0 +1,59 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION: clone, and changing the option from inside the callback
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+include 'server.inc';
+
+$host = curl_cli_server_start();
+
+echo "clone carries the callback, which is then invoked with the clone\n";
+$seen = [];
+$ch = curl_init("{$host}/get.inc?test=method");
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle) use (&$seen): bool {
+    $seen[] = $handle;
+    return true;
+});
+$clone = clone $ch;
+var_dump(curl_exec($clone));
+var_dump($seen !== [] && array_filter($seen, fn ($h) => $h !== $clone) === []);
+
+echo "\nThe clone keeps refusing after the original's callback is removed\n";
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, null);
+curl_setopt($clone, CURLOPT_OPENSOCKETFUNCTION, fn (): bool => false);
+var_dump(curl_exec($clone), curl_errno($clone) === CURLE_COULDNT_CONNECT);
+var_dump(curl_exec($ch));
+
+echo "\nThe callback removes itself and allows the connection\n";
+$calls = 0;
+$ch = curl_init("{$host}/get.inc?test=method");
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 5);
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle) use (&$calls): bool {
+    $calls++;
+    curl_setopt($handle, CURLOPT_OPENSOCKETFUNCTION, null);
+    return true;
+});
+var_dump(curl_exec($ch), $calls);
+var_dump(curl_exec($ch), $calls);
+
+echo "Done\n";
+?>
+--EXPECT--
+clone carries the callback, which is then invoked with the clone
+string(3) "GET"
+bool(true)
+
+The clone keeps refusing after the original's callback is removed
+bool(false)
+bool(true)
+string(3) "GET"
+
+The callback removes itself and allows the connection
+string(3) "GET"
+int(1)
+string(3) "GET"
+int(1)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_file_scheme.phpt b/ext/curl/tests/curl_opensocketfunction_file_scheme.phpt
new file mode 100644
index 000000000000..5c7e09315ddf
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_file_scheme.phpt
@@ -0,0 +1,45 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION is not called for file://, which creates no socket
+--EXTENSIONS--
+curl
+--SKIPIF--
+<?php
+if (!in_array('file', curl_version()['protocols'], true)) die('skip file:// protocol not supported');
+?>
+--FILE--
+<?php
+$file = __DIR__ . '/curl_opensocketfunction_file_scheme.txt';
+file_put_contents($file, "not a socket\n");
+
+$ch = curl_init();
+curl_setopt($ch, CURLOPT_URL, 'file://' . $file);
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+
+$calls = 0;
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () use (&$calls): bool {
+    $calls++;
+    return false;
+});
+
+/* The callback refuses everything, yet the file is still read: the option is not
+ * on its own an SSRF defence, CURLOPT_PROTOCOLS_STR must be used alongside it. */
+var_dump(curl_exec($ch));
+var_dump($calls);
+
+curl_setopt($ch, CURLOPT_PROTOCOLS_STR, 'http,https');
+var_dump(curl_exec($ch));
+var_dump(curl_errno($ch) === CURLE_UNSUPPORTED_PROTOCOL);
+
+echo "Done";
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . '/curl_opensocketfunction_file_scheme.txt');
+?>
+--EXPECT--
+string(13) "not a socket
+"
+int(0)
+bool(false)
+bool(true)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_gc.phpt b/ext/curl/tests/curl_opensocketfunction_gc.phpt
new file mode 100644
index 000000000000..2a7f5f0842ad
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_gc.phpt
@@ -0,0 +1,27 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION: the handle/callback cycle is collectable
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+gc_collect_cycles();
+
+/* The callback closes over the very handle it is registered on, so the handle is
+ * only reclaimable if curl_get_gc() reports the callback to the cycle collector. */
+$ch = curl_init();
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () use ($ch): bool {
+    return true;
+});
+$weak = WeakReference::create($ch);
+unset($ch);
+
+var_dump($weak->get() !== null);
+gc_collect_cycles();
+var_dump($weak->get() === null);
+
+echo "Done";
+?>
+--EXPECT--
+bool(true)
+bool(true)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_multi.phpt b/ext/curl/tests/curl_opensocketfunction_multi.phpt
new file mode 100644
index 000000000000..80b744aedae7
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_multi.phpt
@@ -0,0 +1,59 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION under curl_multi
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+include 'server.inc';
+
+$host = curl_cli_server_start();
+
+$mh = curl_multi_init();
+$handles = [];
+$calls = [];
+
+foreach ([true, false] as $i => $allow) {
+    $ch = curl_init();
+    curl_setopt($ch, CURLOPT_URL, "{$host}/get.inc?test=method");
+    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+    curl_setopt($ch, CURLOPT_FORBID_REUSE, true);
+    $calls[$i] = 0;
+    curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () use ($allow, $i, &$calls): bool {
+        $calls[$i]++;
+        return $allow;
+    });
+    curl_multi_add_handle($mh, $ch);
+    $handles[$i] = $ch;
+}
+
+do {
+    curl_multi_exec($mh, $active);
+    if ($active) {
+        curl_multi_select($mh);
+    }
+} while ($active);
+
+$results = [];
+while ($info = curl_multi_info_read($mh)) {
+    $results[array_search($info['handle'], $handles, true)] = $info['result'];
+}
+ksort($results);
+
+var_dump($calls[0] >= 1, $calls[1] >= 1);
+var_dump(curl_multi_getcontent($handles[0]));
+var_dump($results[0] === CURLE_OK);
+var_dump($results[1] === CURLE_COULDNT_CONNECT);
+
+foreach ($handles as $ch) {
+    curl_multi_remove_handle($mh, $ch);
+}
+
+echo "Done";
+?>
+--EXPECT--
+bool(true)
+bool(true)
+string(3) "GET"
+bool(true)
+bool(true)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_next_address.phpt b/ext/curl/tests/curl_opensocketfunction_next_address.phpt
new file mode 100644
index 000000000000..7c991fb59d96
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_next_address.phpt
@@ -0,0 +1,46 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION: refusing an address makes libcurl try the next one
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+/* No listener is needed: the callback runs before connect() */
+function probe(callable $callback): void {
+    $ch = curl_init('http://example.test:1/');
+    curl_setopt($ch, CURLOPT_RESOLVE, ['example.test:1:127.0.0.2,127.0.0.3']);
+    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+    curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, $callback);
+    try {
+        var_dump(curl_exec($ch));
+    } catch (Exception $e) {
+        echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+    }
+    var_dump(curl_errno($ch) === CURLE_COULDNT_CONNECT);
+}
+
+echo "Refusing every address\n";
+probe(function (CurlHandle $handle, string $address, int $port): bool {
+    echo "$address:$port\n";
+    return false;
+});
+
+echo "\nThrowing on the first address\n";
+probe(function (CurlHandle $handle, string $address, int $port): bool {
+    echo "$address:$port\n";
+    throw new Exception('refused');
+});
+
+echo "Done";
+?>
+--EXPECT--
+Refusing every address
+127.0.0.2:1
+127.0.0.3:1
+bool(false)
+bool(true)
+
+Throwing on the first address
+127.0.0.2:1
+Exception: refused
+bool(true)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_redirect.phpt b/ext/curl/tests/curl_opensocketfunction_redirect.phpt
new file mode 100644
index 000000000000..aec98662d042
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_redirect.phpt
@@ -0,0 +1,61 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION can refuse a redirect hop
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+include 'server.inc';
+
+$host = curl_cli_server_start();
+
+$ch = curl_init();
+curl_setopt($ch, CURLOPT_URL, "{$host}/get.inc?test=redirect&target=method&code=302");
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
+/* Forbid reuse so each hop opens a new connection and is therefore vetted. A
+ * pooled connection creates no socket and is not seen by the callback at all. */
+curl_setopt($ch, CURLOPT_FORBID_REUSE, true);
+
+/* libcurl calls the callback once per socket, so a dual stack host may be
+ * attempted for several address families. The redirect count identifies the hop. */
+$hops = [];
+
+echo "Allowing every hop\n";
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle) use (&$hops): bool {
+    $hops[] = curl_getinfo($handle, CURLINFO_REDIRECT_COUNT) . ' ' . curl_getinfo($handle, CURLINFO_EFFECTIVE_URL);
+    return true;
+});
+var_dump(curl_exec($ch));
+print_r(array_values(array_unique($hops)));
+
+echo "\nRefusing the redirect hop\n";
+$hops = [];
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle) use (&$hops): bool {
+    $redirects = curl_getinfo($handle, CURLINFO_REDIRECT_COUNT);
+    $hops[] = $redirects . ' ' . curl_getinfo($handle, CURLINFO_EFFECTIVE_URL);
+    return $redirects === 0;
+});
+var_dump(curl_exec($ch));
+print_r(array_values(array_unique($hops)));
+var_dump(curl_errno($ch) === CURLE_COULDNT_CONNECT);
+
+echo "Done";
+?>
+--EXPECTF--
+Allowing every hop
+string(3) "GET"
+Array
+(
+    [0] => 0 http://%s/get.inc?test=redirect&target=method&code=302
+    [1] => 1 http://%s/get.inc?test=method
+)
+
+Refusing the redirect hop
+bool(false)
+Array
+(
+    [0] => 0 http://%s/get.inc?test=redirect&target=method&code=302
+    [1] => 1 http://%s/get.inc?test=method
+)
+bool(true)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_reuse.phpt b/ext/curl/tests/curl_opensocketfunction_reuse.phpt
new file mode 100644
index 000000000000..10452d035550
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_reuse.phpt
@@ -0,0 +1,67 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION is not invoked for a connection reused from the pool
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+include 'keepalive_server.inc';
+
+/* The callback is invoked when a socket is created. A connection taken from
+ * libcurl's pool creates none, so the callback only vets connections opened
+ * while it is set. The server answers with the number of connections it has
+ * accepted, which shows whether a request was sent on a new connection. */
+$url = curl_keepalive_server_start() . '/';
+$calls = 0;
+$count = function (CurlHandle $handle, string $address, int $port) use (&$calls): bool {
+    $calls++;
+    return true;
+};
+
+echo "Set before the first transfer: only the new connection is vetted\n";
+$ch = curl_init($url);
+curl_setopt_array($ch, [CURLOPT_RETURNTRANSFER => true, CURLOPT_PROXY => '']);
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, $count);
+var_dump(curl_exec($ch), curl_exec($ch), $calls);
+
+echo "\nSet after a transfer: the pooled connection is reused without the callback\n";
+$calls = 0;
+$ch = curl_init($url);
+curl_setopt_array($ch, [CURLOPT_RETURNTRANSFER => true, CURLOPT_PROXY => '']);
+var_dump(curl_exec($ch));
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, $count);
+var_dump(curl_exec($ch), $calls);
+
+echo "\nCURLOPT_FRESH_CONNECT opens a new connection, which is vetted\n";
+curl_setopt($ch, CURLOPT_FRESH_CONNECT, true);
+var_dump(curl_exec($ch), $calls);
+
+echo "\nCURLOPT_PREREQFUNCTION runs before every request, on a reused connection too\n";
+curl_setopt($ch, CURLOPT_FRESH_CONNECT, false);
+curl_setopt($ch, CURLOPT_PREREQFUNCTION, function (CurlHandle $handle, string $ip, string $localIp, int $port, int $localPort): int {
+    var_dump($ip);
+    return CURL_PREREQFUNC_OK;
+});
+var_dump(curl_exec($ch), $calls);
+
+echo "Done\n";
+?>
+--EXPECT--
+Set before the first transfer: only the new connection is vetted
+string(1) "1"
+string(1) "1"
+int(1)
+
+Set after a transfer: the pooled connection is reused without the callback
+string(1) "2"
+string(1) "2"
+int(0)
+
+CURLOPT_FRESH_CONNECT opens a new connection, which is vetted
+string(1) "3"
+int(1)
+
+CURLOPT_PREREQFUNCTION runs before every request, on a reused connection too
+string(9) "127.0.0.1"
+string(1) "3"
+int(1)
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_share_free.phpt b/ext/curl/tests/curl_opensocketfunction_share_free.phpt
new file mode 100644
index 000000000000..cdaaebeb1b4d
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_share_free.phpt
@@ -0,0 +1,63 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION: freeing the handle while its connection stays pooled
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+include 'keepalive_server.inc';
+
+$host = curl_keepalive_server_start();
+
+/* The callback holds a reference to the handle it is set on: releasing the easy
+ * handle while its connection stays cached in a share or a multi handle must stay
+ * clean. The server keeps connections open and answers with the number it has
+ * accepted, so the responses show that the connection really was pooled and
+ * reused. The point of this test is to exercise the teardown order under
+ * ASAN/valgrind in CI. */
+$sh = curl_share_init();
+curl_share_setopt($sh, CURLSHOPT_SHARE, CURL_LOCK_DATA_CONNECT);
+
+for ($i = 0; $i < 3; $i++) {
+    $ch = curl_init();
+    curl_setopt($ch, CURLOPT_URL, "{$host}/");
+    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+    curl_setopt($ch, CURLOPT_PROXY, '');
+    curl_setopt($ch, CURLOPT_SHARE, $sh);
+    curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () use ($ch): bool {
+        return true;
+    });
+    var_dump(curl_exec($ch));
+    unset($ch);
+}
+
+unset($sh);
+
+$mh = curl_multi_init();
+$ch = curl_init();
+curl_setopt($ch, CURLOPT_URL, "{$host}/");
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+curl_setopt($ch, CURLOPT_PROXY, '');
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () use ($ch): bool {
+    return true;
+});
+curl_multi_add_handle($mh, $ch);
+
+do {
+    curl_multi_exec($mh, $active);
+    if ($active) {
+        curl_multi_select($mh);
+    }
+} while ($active);
+
+var_dump(curl_multi_getcontent($ch));
+curl_multi_remove_handle($mh, $ch);
+unset($ch, $mh);
+
+echo "Done";
+?>
+--EXPECT--
+string(1) "1"
+string(1) "1"
+string(1) "1"
+string(1) "2"
+Done
diff --git a/ext/curl/tests/curl_opensocketfunction_unix_socket.phpt b/ext/curl/tests/curl_opensocketfunction_unix_socket.phpt
new file mode 100644
index 000000000000..143e8edeba9a
--- /dev/null
+++ b/ext/curl/tests/curl_opensocketfunction_unix_socket.phpt
@@ -0,0 +1,38 @@
+--TEST--
+CURLOPT_OPENSOCKETFUNCTION reports the path and port 0 for a UNIX socket
+--EXTENSIONS--
+curl
+--SKIPIF--
+<?php
+if (!defined('CURLOPT_UNIX_SOCKET_PATH')) die('skip CURLOPT_UNIX_SOCKET_PATH not available');
+if (PHP_OS_FAMILY === 'Windows') die('skip UNIX domain sockets');
+?>
+--FILE--
+<?php
+/* A path has no port: $port is 0, as CURLINFO_PRIMARY_PORT reports. The path is
+ * kept short, as sun_path holds 104 bytes on some systems; no socket is
+ * created, since the callback refuses it. */
+$path = sys_get_temp_dir() . '/php-curl-' . getmypid() . '.sock';
+if (strlen($path) > 100) {
+    $path = '/tmp/php-curl-' . getmypid() . '.sock';
+}
+
+$ch = curl_init();
+curl_setopt($ch, CURLOPT_URL, 'http://localhost/');
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+curl_setopt($ch, CURLOPT_UNIX_SOCKET_PATH, $path);
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle, string $address, int $port) use ($path): bool {
+    var_dump($address === $path, $port);
+    return false;
+});
+var_dump(curl_exec($ch));
+var_dump(curl_errno($ch) === CURLE_COULDNT_CONNECT);
+
+echo "Done";
+?>
+--EXPECT--
+bool(true)
+int(0)
+bool(false)
+bool(true)
+Done
diff --git a/ext/curl/tests/curl_setopt_CURLOPT_OPENSOCKETFUNCTION.phpt b/ext/curl/tests/curl_setopt_CURLOPT_OPENSOCKETFUNCTION.phpt
new file mode 100644
index 000000000000..ec8c7397d30e
--- /dev/null
+++ b/ext/curl/tests/curl_setopt_CURLOPT_OPENSOCKETFUNCTION.phpt
@@ -0,0 +1,249 @@
+--TEST--
+Curl option CURLOPT_OPENSOCKETFUNCTION
+--EXTENSIONS--
+curl
+--FILE--
+<?php
+include 'server.inc';
+
+$host = curl_cli_server_start();
+$port = (int) (explode(':', $host))[1];
+
+/* libcurl invokes the callback once per socket it creates, so a dual stack host
+ * may be attempted for more than one address family. Every assertion below is
+ * therefore made over all the attempts, never on their number. */
+function all_handles_are(array $seen, CurlHandle $expected): bool {
+    if ($seen === []) {
+        return false;
+    }
+    foreach ($seen as $handle) {
+        if ($handle !== $expected) {
+            return false;
+        }
+    }
+    return true;
+}
+
+var_dump(CURLOPT_OPENSOCKETFUNCTION);
+
+$ch = curl_init();
+curl_setopt($ch, CURLOPT_URL, "{$host}/get.inc");
+curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+
+echo "\nAllowing the connection\n";
+$seen = [];
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle, string $address, int $port) use (&$seen): bool {
+    $seen[] = [$address, $port];
+    return true;
+});
+var_dump(curl_exec($ch));
+var_dump(curl_errno($ch));
+/* The test server listens on localhost, so whichever family libcurl tries the
+ * address is one of the two loopback literals. Checked without ext/filter so that
+ * curl alone is enough to run this test. */
+$ok = $seen !== [];
+foreach ($seen as [$address, $p]) {
+    $ok = $ok
+        && in_array($address, ['127.0.0.1', '::1'], true)
+        && $p === $port;
+}
+var_dump($ok);
+
+echo "\nRefusing the connection\n";
+$refuse = function (): bool { return false; };
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, $refuse);
+var_dump(curl_exec($ch));
+var_dump(curl_errno($ch) === CURLE_COULDNT_CONNECT);
+
+echo "\nTesting with curl_copy_handle\n";
+/* The copy must be given its own callback, invoked with the copy. */
+$invoked = [];
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (CurlHandle $handle) use (&$invoked): bool {
+    $invoked[] = $handle;
+    return true;
+});
+var_dump(curl_exec($ch));
+var_dump(all_handles_are($invoked, $ch));
+$ch2 = curl_copy_handle($ch);
+$invoked = [];
+var_dump(curl_exec($ch2));
+var_dump(all_handles_are($invoked, $ch2));
+var_dump(curl_errno($ch2));
+curl_setopt($ch2, CURLOPT_OPENSOCKETFUNCTION, $refuse);
+var_dump(curl_exec($ch2));
+var_dump(curl_errno($ch2) === CURLE_COULDNT_CONNECT);
+unset($ch2);
+
+echo "\nTesting with curl_reset\n";
+$ch3 = curl_copy_handle($ch);
+$cb3 = function (): bool { return false; };
+curl_setopt($ch3, CURLOPT_OPENSOCKETFUNCTION, $cb3);
+$weak = WeakReference::create($cb3);
+unset($cb3);
+curl_reset($ch3);
+/* curl_reset() must release the callback, not merely clear the libcurl option. */
+var_dump($weak->get() === null);
+curl_setopt($ch3, CURLOPT_URL, "{$host}/get.inc");
+curl_setopt($ch3, CURLOPT_RETURNTRANSFER, true);
+var_dump(curl_exec($ch3));
+unset($ch3);
+
+echo "\nTesting with no return value\n";
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () {});
+try {
+    curl_exec($ch);
+} catch (\TypeError $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+
+echo "\nTesting with an invalid return type\n";
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () { return 1; });
+try {
+    curl_exec($ch);
+} catch (\TypeError $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+
+echo "\nTesting with an exception thrown from the callback\n";
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function () {
+    throw new \Exception('refused');
+});
+try {
+    curl_exec($ch);
+} catch (\Exception $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+var_dump(curl_errno($ch) === CURLE_COULDNT_CONNECT);
+
+echo "\nTesting with an exception thrown by a destructor after returning true\n";
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, function (): bool {
+    $o = new class { function __destruct() { throw new \Exception('refused by destructor'); } };
+    return true;
+});
+try {
+    curl_exec($ch);
+} catch (\Exception $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+var_dump(curl_errno($ch) === CURLE_COULDNT_CONNECT);
+
+echo "\nTesting with an invalid option value\n";
+try {
+    curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, 42);
+} catch (\TypeError $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+
+echo "\nTesting with an invalid option callback\n";
+try {
+    curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, 'function_does_not_exist');
+} catch (\TypeError $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+
+echo "\nTesting through curl_setopt_array()\n";
+try {
+    curl_setopt_array($ch, [CURLOPT_OPENSOCKETFUNCTION => 42]);
+} catch (\TypeError $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+
+echo "\nTesting a rejected callable followed by curl_copy_handle()\n";
+$src = curl_init();
+curl_setopt($src, CURLOPT_URL, "{$host}/get.inc");
+curl_setopt($src, CURLOPT_RETURNTRANSFER, true);
+curl_setopt($src, CURLOPT_OPENSOCKETFUNCTION, $refuse);
+try {
+    curl_setopt($src, CURLOPT_OPENSOCKETFUNCTION, 'function_does_not_exist');
+} catch (\TypeError $e) {
+    echo $e::class, PHP_EOL;
+}
+/* A rejected callable also releases the previously installed handler, so the
+ * option is left disarmed and libcurl creates the socket itself again. The copy
+ * must not carry a CURLOPT_OPENSOCKETDATA pointer to the source handle, which
+ * would dangle as soon as the source is released. */
+$copy = curl_copy_handle($src);
+unset($src);
+var_dump(curl_exec($copy));
+unset($copy);
+
+echo "\nTesting with null as the callback\n";
+$cbn = function (): bool { return false; };
+curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, $cbn);
+$weakNull = WeakReference::create($cbn);
+unset($cbn);
+var_dump(curl_exec($ch));
+var_dump(curl_setopt($ch, CURLOPT_OPENSOCKETFUNCTION, null));
+/* null must release the callback and hand socket creation back to libcurl. */
+var_dump($weakNull->get() === null);
+var_dump(curl_exec($ch));
+var_dump(curl_errno($ch));
+
+echo "\nDone";
+?>
+--EXPECT--
+int(20163)
+
+Allowing the connection
+string(25) "Hello World!
+Hello World!"
+int(0)
+bool(true)
+
+Refusing the connection
+bool(false)
+bool(true)
+
+Testing with curl_copy_handle
+string(25) "Hello World!
+Hello World!"
+bool(true)
+string(25) "Hello World!
+Hello World!"
+bool(true)
+int(0)
+bool(false)
+bool(true)
+
+Testing with curl_reset
+bool(true)
+string(25) "Hello World!
+Hello World!"
+
+Testing with no return value
+TypeError: The CURLOPT_OPENSOCKETFUNCTION callback must return a bool
+
+Testing with an invalid return type
+TypeError: The CURLOPT_OPENSOCKETFUNCTION callback must return a bool
+
+Testing with an exception thrown from the callback
+Exception: refused
+bool(true)
+
+Testing with an exception thrown by a destructor after returning true
+Exception: refused by destructor
+bool(true)
+
+Testing with an invalid option value
+TypeError: curl_setopt(): Argument #3 ($value) must be a valid callback for option CURLOPT_OPENSOCKETFUNCTION, no array or string given
+
+Testing with an invalid option callback
+TypeError: curl_setopt(): Argument #3 ($value) must be a valid callback for option CURLOPT_OPENSOCKETFUNCTION, function "function_does_not_exist" not found or invalid function name
+
+Testing through curl_setopt_array()
+TypeError: curl_setopt_array(): Argument #2 ($options) must be a valid callback for option CURLOPT_OPENSOCKETFUNCTION, no array or string given
+
+Testing a rejected callable followed by curl_copy_handle()
+TypeError
+string(25) "Hello World!
+Hello World!"
+
+Testing with null as the callback
+bool(false)
+bool(true)
+bool(true)
+string(25) "Hello World!
+Hello World!"
+int(0)
+
+Done
diff --git a/ext/curl/tests/keepalive_server.inc b/ext/curl/tests/keepalive_server.inc
new file mode 100644
index 000000000000..d81dc57e4df7
--- /dev/null
+++ b/ext/curl/tests/keepalive_server.inc
@@ -0,0 +1,63 @@
+<?php
+/* A minimal HTTP/1.1 server that keeps connections open, unlike the CLI server
+ * started by server.inc, so that tests can observe libcurl's connection reuse.
+ * Each response body is the number of connections accepted so far: two
+ * responses with the same body were served on the same connection. */
+function curl_keepalive_server_start(): string {
+    $code = <<<'PHP'
+    $server = stream_socket_server('tcp://127.0.0.1:0', $errno, $errstr);
+    if ($server === false) {
+        echo "error: $errstr\n";
+        exit(1);
+    }
+    echo stream_socket_get_name($server, false), "\n";
+    $clients = [];
+    $buffers = [];
+    $accepted = 0;
+    while (true) {
+        $read = $clients;
+        $read[] = $server;
+        $write = $except = null;
+        if (stream_select($read, $write, $except, 60) < 1) {
+            break;
+        }
+        foreach ($read as $socket) {
+            if ($socket === $server) {
+                $client = stream_socket_accept($server);
+                $accepted++;
+                $clients[(int) $client] = $client;
+                $buffers[(int) $client] = '';
+                continue;
+            }
+            $data = fread($socket, 8192);
+            if ($data === '' || $data === false) {
+                unset($clients[(int) $socket], $buffers[(int) $socket]);
+                fclose($socket);
+                continue;
+            }
+            $buffers[(int) $socket] .= $data;
+            while (($end = strpos($buffers[(int) $socket], "\r\n\r\n")) !== false) {
+                $buffers[(int) $socket] = substr($buffers[(int) $socket], $end + 4);
+                $body = (string) $accepted;
+                fwrite($socket, "HTTP/1.1 200 OK\r\nContent-Length: " . strlen($body) . "\r\n\r\n" . $body);
+            }
+        }
+    }
+    PHP;
+
+    $php = getenv('TEST_PHP_EXECUTABLE') ?: PHP_BINARY;
+    $process = proc_open([$php, '-n', '-r', $code], [1 => ['pipe', 'w']], $pipes);
+    $address = trim((string) fgets($pipes[1]));
+    if (!preg_match('/^127\.0\.0\.1:\d+$/', $address)) {
+        proc_terminate($process);
+        die("Failed to start the keep-alive server: $address");
+    }
+
+    register_shutdown_function(function () use ($process, $pipes): void {
+        proc_terminate($process);
+        fclose($pipes[1]);
+        proc_close($process);
+    });
+
+    return "http://$address";
+}