diff --git a/Zend/zend_alloc.c b/Zend/zend_alloc.c
index 2b52babc98ee..d0c6f751e518 100644
--- a/Zend/zend_alloc.c
+++ b/Zend/zend_alloc.c
@@ -1825,6 +1825,10 @@ static zend_never_inline void *zend_mm_realloc_huge(zend_mm_heap *heap, void *pt
 #if ZEND_DEBUG
 	real_size = size;
 	size = ZEND_MM_ALIGNED_SIZE(size) + ZEND_MM_ALIGNED_SIZE(sizeof(zend_mm_debug_info));
+	if (UNEXPECTED(size < real_size)) {
+		zend_mm_delayed_error_or_panic(heap->fallible, "Possible integer overflow in memory allocation (%zu + %zu)", ZEND_MM_ALIGNED_SIZE(real_size), ZEND_MM_ALIGNED_SIZE(sizeof(zend_mm_debug_info)));
+		return NULL;
+	}
 #endif
 	if (size > ZEND_MM_MAX_LARGE_SIZE) {
 #if ZEND_DEBUG
@@ -1835,10 +1839,15 @@ static zend_never_inline void *zend_mm_realloc_huge(zend_mm_heap *heap, void *pt
 		 * We allocate them with 2MB size granularity, to avoid many
 		 * reallocations when they are extended by small pieces
 		 */
-		new_size = ZEND_MM_ALIGNED_SIZE_EX(size, MAX(REAL_PAGE_SIZE, ZEND_MM_CHUNK_SIZE));
+		size_t alignment = MAX(REAL_PAGE_SIZE, ZEND_MM_CHUNK_SIZE);
 #else
-		new_size = ZEND_MM_ALIGNED_SIZE_EX(size, REAL_PAGE_SIZE);
+		size_t alignment = REAL_PAGE_SIZE;
 #endif
+		new_size = ZEND_MM_ALIGNED_SIZE_EX(size, alignment);
+		if (UNEXPECTED(new_size < size)) {
+			zend_mm_delayed_error_or_panic(heap->fallible, "Possible integer overflow in memory allocation (%zu + %zu)", size, alignment);
+			return NULL;
+		}
 		if (new_size == old_size) {
 #if ZEND_DEBUG
 			zend_mm_change_huge_block_size(heap, ptr, new_size, real_size ZEND_FILE_LINE_RELAY_CC ZEND_FILE_LINE_ORIG_RELAY_CC);
@@ -1920,6 +1929,10 @@ static zend_always_inline void *zend_mm_realloc_heap(zend_mm_heap *heap, void *p
 		size_t real_size = size;
 
 		size = ZEND_MM_ALIGNED_SIZE(size) + ZEND_MM_ALIGNED_SIZE(sizeof(zend_mm_debug_info));
+		if (UNEXPECTED(size < real_size)) {
+			zend_mm_delayed_error_or_panic(heap->fallible, "Possible integer overflow in memory allocation (%zu + %zu)", ZEND_MM_ALIGNED_SIZE(real_size), ZEND_MM_ALIGNED_SIZE(sizeof(zend_mm_debug_info)));
+			return NULL;
+		}
 #endif
 
 		ZEND_MM_CHECK(chunk->heap == heap, "zend_mm_heap corrupted");
diff --git a/Zend/zend_string.h b/Zend/zend_string.h
index 5bd805aa8ddb..91edd5b5ba15 100644
--- a/Zend/zend_string.h
+++ b/Zend/zend_string.h
@@ -131,6 +131,16 @@ static zend_always_inline zend_string *ZSTR_KNOWN(size_t idx) {
 #define ZSTR_MAX_OVERHEAD (ZEND_MM_ALIGNED_SIZE(_ZSTR_HEADER_SIZE + 1))
 #define ZSTR_MAX_LEN (SIZE_MAX - ZSTR_MAX_OVERHEAD)
 
+/* Offset for safe_*alloc(n, m, offset) to size the string like zend_string_alloc(n * m + l). */
+static zend_always_inline size_t zend_string_safe_offset(size_t n, size_t m, size_t l)
+{
+	if (UNEXPECTED(l > ZSTR_MAX_LEN)) {
+		return SIZE_MAX; /* Rejected by the allocator. */
+	}
+	/* Works even if n * m wraps, allocator detects that overflow. */
+	return ZEND_MM_ALIGNED_SIZE(_ZSTR_STRUCT_SIZE(n * m + l)) - n * m;
+}
+
 #define ZSTR_ALLOCA_ALLOC(str, _len, use_heap) do { \
 	(str) = (zend_string *)do_alloca(ZEND_MM_ALIGNED_SIZE_EX(_ZSTR_STRUCT_SIZE(_len), 8), (use_heap)); \
 	GC_SET_REFCOUNT(str, 1); \
@@ -209,10 +219,12 @@ static zend_always_inline zend_string *zend_string_alloc(size_t len, bool persis
 /* Fallible variant of zend_string_safe_alloc */
 static zend_always_inline zend_string *zend_string_safe_falloc(size_t n, size_t m, size_t l, bool persistent, bool fallible)
 {
-	zend_string *ret = (zend_string *)safe_fpemalloc(n, m, ZEND_MM_ALIGNED_SIZE(_ZSTR_STRUCT_SIZE(l)), persistent, fallible);
+	zend_string *ret = (zend_string *)safe_fpemalloc(n, m, zend_string_safe_offset(n, m, l), persistent, fallible);
 	if (ZEND_CONST_COND(fallible, true) && UNEXPECTED(ret == NULL)) {
 		return ret;
 	}
+	/* GCC can't see that allocation fails if l is too large, so make it explicit. */
+	ZEND_ASSERT(l <= ZSTR_MAX_LEN);
 
 	GC_SET_REFCOUNT(ret, 1);
 	GC_TYPE_INFO(ret) = GC_STRING | ((persistent ? IS_STR_PERSISTENT : 0) << GC_FLAGS_SHIFT);
@@ -394,10 +406,12 @@ static zend_always_inline zend_string *zend_string_safe_frealloc(zend_string *s,
 
 	if (!ZSTR_IS_INTERNED(s)) {
 		if (GC_REFCOUNT(s) == 1) {
-			ret = (zend_string *)safe_fperealloc(s, n, m, ZEND_MM_ALIGNED_SIZE(_ZSTR_STRUCT_SIZE(l)), persistent, fallible);
+			ret = (zend_string *)safe_fperealloc(s, n, m, zend_string_safe_offset(n, m, l), persistent, fallible);
 			if (ZEND_CONST_COND(fallible, true) && !ret) {
 				return NULL;
 			}
+			/* GCC can't see that allocation fails if l is too large, so make it explicit. */
+			ZEND_ASSERT(l <= ZSTR_MAX_LEN);
 			ZSTR_LEN(ret) = (n * m) + l;
 			zend_string_forget_hash_val(ret);
 			return ret;
diff --git a/ext/mbstring/libmbfl/mbfl/mbfl_encoding.h b/ext/mbstring/libmbfl/mbfl/mbfl_encoding.h
index e71737318bf7..594e2f3ebd2a 100644
--- a/ext/mbstring/libmbfl/mbfl/mbfl_encoding.h
+++ b/ext/mbstring/libmbfl/mbfl/mbfl_encoding.h
@@ -155,7 +155,7 @@ typedef zend_string* (*mb_cut_fn)(unsigned char *str, size_t from, size_t len, u
 static inline void mb_convert_buf_init(mb_convert_buf *buf, size_t initsize, uint32_t repl_char, unsigned int err_mode)
 {
 	buf->state = buf->errors = 0;
-	buf->str = emalloc(_ZSTR_STRUCT_SIZE(initsize));
+	buf->str = emalloc(ZEND_MM_ALIGNED_SIZE(_ZSTR_STRUCT_SIZE(initsize)));
 	buf->out = (unsigned char*)ZSTR_VAL(buf->str);
 	buf->limit = buf->out + initsize;
 	buf->replacement_char = repl_char;
@@ -167,16 +167,18 @@ static inline zend_result mb_convert_buf_ensure(mb_convert_buf *buf, unsigned ch
 	ZEND_ASSERT(*out <= *limit);
 	if ((size_t)(*limit - *out) < needed) {
 		size_t oldsize = *limit - (unsigned char*)ZSTR_VAL((buf)->str);
-		size_t newsize = oldsize + MAX(oldsize >> 1, needed);
-		zend_string *newstr = safe_ferealloc((buf)->str, 1, newsize, _ZSTR_STRUCT_SIZE(0), /* fallible */ true);
+		size_t growth = MAX(oldsize >> 1, needed);
+		zend_string *newstr = safe_ferealloc((buf)->str, 1, oldsize, zend_string_safe_offset(1, oldsize, growth), /* fallible */ true);
 		if (!newstr) {
 			/* An earlier growth may have moved the buffer. */
 			buf->out = *out;
 			buf->limit = *limit;
 			return FAILURE;
 		}
+		/* GCC can't see that allocation fails if growth is too large, so make it explicit. */
+		ZEND_ASSERT(growth <= ZSTR_MAX_LEN);
 		*out = (unsigned char*)ZSTR_VAL(newstr) + (*out - (unsigned char*)ZSTR_VAL((buf)->str));
-		*limit = (unsigned char*)ZSTR_VAL(newstr) + newsize;
+		*limit = (unsigned char*)ZSTR_VAL(newstr) + oldsize + growth;
 		(buf)->str = newstr;
 	}
 	return SUCCESS;
diff --git a/ext/standard/tests/strings/chunk_split_variation2_32bit.phpt b/ext/standard/tests/strings/chunk_split_variation2_32bit.phpt
index c83a37f00edb..1f378a739d53 100644
--- a/ext/standard/tests/strings/chunk_split_variation2_32bit.phpt
+++ b/ext/standard/tests/strings/chunk_split_variation2_32bit.phpt
@@ -16,4 +16,4 @@ var_dump(chunk_split($a,$b,$c));
 --EXPECTF--
 *** Testing chunk_split() : unexpected large 'end' string argument variation 2 ***
 
-Fatal error: Possible integer overflow in memory allocation (65537 * 65537 + %r65556|65560%r) in %s on line %d
+Fatal error: Possible integer overflow in memory allocation (65537 * 65537 + %d) in %s on line %d