diff --git a/UPGRADING.INTERNALS b/UPGRADING.INTERNALS
index 3fd2f27f25270..0e6528ae15248 100644
--- a/UPGRADING.INTERNALS
+++ b/UPGRADING.INTERNALS
@@ -148,3 +148,7 @@ PHP 8.5 INTERNALS UPGRADE NOTES
 ========================
 5. SAPI changes
 ========================
+
+- SAPIs must now call php_child_init() after a fork. If php-src code was
+  executed in other threads than the one initiating the fork,
+  refresh_memory_manager() must be called in every such thread.
diff --git a/Zend/zend_alloc.c b/Zend/zend_alloc.c
index f2f801db63396..edadf024df24b 100644
--- a/Zend/zend_alloc.c
+++ b/Zend/zend_alloc.c
@@ -317,7 +317,9 @@ struct _zend_mm_heap {
 		} debug;
 	};
 #endif
+#if ZEND_DEBUG
 	pid_t pid;
+#endif
 	zend_random_bytes_insecure_state rand_state;
 };
 
@@ -1310,15 +1312,20 @@ static zend_always_inline zend_mm_free_slot* zend_mm_encode_free_slot(const zend
 #endif
 }
 
-static zend_always_inline zend_mm_free_slot* zend_mm_decode_free_slot(zend_mm_heap *heap, zend_mm_free_slot *slot)
+static zend_always_inline zend_mm_free_slot* zend_mm_decode_free_slot_key(uintptr_t shadow_key, zend_mm_free_slot *slot)
 {
 #ifdef WORDS_BIGENDIAN
-	return (zend_mm_free_slot*)((uintptr_t)slot ^ heap->shadow_key);
+	return (zend_mm_free_slot*)((uintptr_t)slot ^ shadow_key);
 #else
-	return (zend_mm_free_slot*)(BSWAPPTR((uintptr_t)slot ^ heap->shadow_key));
+	return (zend_mm_free_slot*)(BSWAPPTR((uintptr_t)slot ^ shadow_key));
 #endif
 }
 
+static zend_always_inline zend_mm_free_slot* zend_mm_decode_free_slot(zend_mm_heap *heap, zend_mm_free_slot *slot)
+{
+	return zend_mm_decode_free_slot_key(heap->shadow_key, slot);
+}
+
 static zend_always_inline void zend_mm_set_next_free_slot(zend_mm_heap *heap, uint32_t bin_num, zend_mm_free_slot *slot, zend_mm_free_slot *next)
 {
 	ZEND_ASSERT(bin_data_size[bin_num] >= ZEND_MM_MIN_USEABLE_BIN_SIZE);
@@ -2027,6 +2034,34 @@ static void zend_mm_init_key(zend_mm_heap *heap)
 	zend_mm_refresh_key(heap);
 }
 
+ZEND_API void zend_mm_refresh_key_child(zend_mm_heap *heap)
+{
+	uintptr_t old_key = heap->shadow_key;
+
+	zend_mm_init_key(heap);
+
+	/* Update shadow pointers with new key */
+	for (int i = 0; i < ZEND_MM_BINS; i++) {
+		zend_mm_free_slot *slot = heap->free_slot[i];
+		if (!slot) {
+			continue;
+		}
+		zend_mm_free_slot *next;
+		while ((next = slot->next_free_slot)) {
+			zend_mm_free_slot *shadow = ZEND_MM_FREE_SLOT_PTR_SHADOW(slot, i);
+			if (UNEXPECTED(next != zend_mm_decode_free_slot_key(old_key, shadow))) {
+				zend_mm_panic("zend_mm_heap corrupted");
+			}
+			zend_mm_set_next_free_slot(heap, i, slot, next);
+			slot = next;
+		}
+	}
+
+#if ZEND_DEBUG
+	heap->pid = getpid();
+#endif
+}
+
 static zend_mm_heap *zend_mm_init(void)
 {
 	zend_mm_chunk *chunk = (zend_mm_chunk*)zend_mm_chunk_alloc_int(ZEND_MM_CHUNK_SIZE, ZEND_MM_CHUNK_SIZE);
@@ -2075,7 +2110,9 @@ static zend_mm_heap *zend_mm_init(void)
 	heap->storage = NULL;
 #endif
 	heap->huge_list = NULL;
+#if ZEND_DEBUG
 	heap->pid = getpid();
+#endif
 	return heap;
 }
 
@@ -2535,13 +2572,12 @@ ZEND_API void zend_mm_shutdown(zend_mm_heap *heap, bool full, bool silent)
 		p->free_map[0] = (1L << ZEND_MM_FIRST_PAGE) - 1;
 		p->map[0] = ZEND_MM_LRUN(ZEND_MM_FIRST_PAGE);
 
-		pid_t pid = getpid();
-		if (heap->pid != pid) {
-			zend_mm_init_key(heap);
-			heap->pid = pid;
-		} else {
-			zend_mm_refresh_key(heap);
-		}
+#if ZEND_DEBUG
+		ZEND_ASSERT(getpid() == heap->pid
+				&& "heap was re-used without calling zend_mm_refresh_key_child() after a fork");
+#endif
+
+		zend_mm_refresh_key(heap);
 	}
 }
 
@@ -2949,6 +2985,11 @@ ZEND_API void shutdown_memory_manager(bool silent, bool full_shutdown)
 	zend_mm_shutdown(AG(mm_heap), full_shutdown, silent);
 }
 
+ZEND_API void refresh_memory_manager(void)
+{
+	zend_mm_refresh_key_child(AG(mm_heap));
+}
+
 static ZEND_COLD ZEND_NORETURN void zend_out_of_memory(void)
 {
 	fprintf(stderr, "Out of memory\n");
@@ -3506,7 +3547,9 @@ ZEND_API zend_mm_heap *zend_mm_startup_ex(const zend_mm_handlers *handlers, void
 		memcpy(storage->data, data, data_size);
 	}
 	heap->storage = storage;
+#if ZEND_DEBUG
 	heap->pid = getpid();
+#endif
 	return heap;
 #else
 	return NULL;
diff --git a/Zend/zend_alloc.h b/Zend/zend_alloc.h
index 541989a2a13e0..264e13848d1b7 100644
--- a/Zend/zend_alloc.h
+++ b/Zend/zend_alloc.h
@@ -220,6 +220,7 @@ ZEND_API bool zend_alloc_in_memory_limit_error_reporting(void);
 
 ZEND_API void start_memory_manager(void);
 ZEND_API void shutdown_memory_manager(bool silent, bool full_shutdown);
+ZEND_API void refresh_memory_manager(void);
 ZEND_API bool is_zend_mm(void);
 ZEND_API bool is_zend_ptr(const void *ptr);
 
@@ -316,6 +317,8 @@ struct _zend_mm_storage {
 ZEND_API zend_mm_storage *zend_mm_get_storage(zend_mm_heap *heap);
 ZEND_API zend_mm_heap *zend_mm_startup_ex(const zend_mm_handlers *handlers, void *data, size_t data_size);
 
+ZEND_API void zend_mm_refresh_key_child(zend_mm_heap *heap);
+
 /*
 
 // The following example shows how to use zend_mm_heap API with custom storage
diff --git a/ext/pcntl/pcntl.c b/ext/pcntl/pcntl.c
index e11de304b1b79..4d1b5dff2e353 100644
--- a/ext/pcntl/pcntl.c
+++ b/ext/pcntl/pcntl.c
@@ -32,6 +32,7 @@
 #include "php_signal.h"
 #include "php_ticks.h"
 #include "zend_fibers.h"
+#include "main/php_main.h"
 
 #if defined(HAVE_GETPRIORITY) || defined(HAVE_SETPRIORITY) || defined(HAVE_WAIT3)
 #include <sys/wait.h>
@@ -297,7 +298,7 @@ PHP_FUNCTION(pcntl_fork)
 
 		}
 	} else if (id == 0) {
-		zend_max_execution_timer_init();
+		php_child_init();
 	}
 
 	RETURN_LONG((zend_long) id);
@@ -1560,7 +1561,7 @@ PHP_FUNCTION(pcntl_rfork)
 			php_error_docref(NULL, E_WARNING, "Error %d", errno);
 		}
 	} else if (pid == 0) {
-		zend_max_execution_timer_init();
+		php_child_init();
 	}
 
 	RETURN_LONG((zend_long) pid);
@@ -1605,7 +1606,7 @@ PHP_FUNCTION(pcntl_forkx)
 			php_error_docref(NULL, E_WARNING, "Error %d", errno);
 		}
 	} else if (pid == 0) {
-		zend_max_execution_timer_init();
+		php_child_init();
 	}
 
 	RETURN_LONG((zend_long) pid);
diff --git a/main/main.c b/main/main.c
index 3518e4137ecef..e2973f17c248c 100644
--- a/main/main.c
+++ b/main/main.c
@@ -1816,6 +1816,12 @@ static void sigchld_handler(int apar)
 /* }}} */
 #endif
 
+PHPAPI void php_child_init(void)
+{
+	refresh_memory_manager();
+	zend_max_execution_timer_init();
+}
+
 /* {{{ php_request_startup */
 zend_result php_request_startup(void)
 {
diff --git a/main/php_main.h b/main/php_main.h
index a5b049487db23..bd28a0dee1d7f 100644
--- a/main/php_main.h
+++ b/main/php_main.h
@@ -49,6 +49,7 @@ ZEND_ATTRIBUTE_CONST PHPAPI const char *php_build_provider(void);
 PHPAPI char *php_get_version(sapi_module_struct *sapi_module);
 PHPAPI void php_print_version(sapi_module_struct *sapi_module);
 
+PHPAPI void php_child_init(void);
 PHPAPI zend_result php_request_startup(void);
 PHPAPI void php_request_shutdown(void *dummy);
 PHPAPI zend_result php_module_startup(sapi_module_struct *sf, zend_module_entry *additional_module);
diff --git a/sapi/apache2handler/sapi_apache2.c b/sapi/apache2handler/sapi_apache2.c
index e87223b055e12..88fc584a8bcbb 100644
--- a/sapi/apache2handler/sapi_apache2.c
+++ b/sapi/apache2handler/sapi_apache2.c
@@ -752,6 +752,7 @@ zend_first_try {
 static void php_apache_child_init(apr_pool_t *pchild, server_rec *s)
 {
 	apr_pool_cleanup_register(pchild, NULL, php_apache_child_shutdown, apr_pool_cleanup_null);
+	php_child_init();
 }
 
 #ifdef ZEND_SIGNALS
diff --git a/sapi/cgi/cgi_main.c b/sapi/cgi/cgi_main.c
index e3cd6f49b9f0c..6db96a43ac97b 100644
--- a/sapi/cgi/cgi_main.c
+++ b/sapi/cgi/cgi_main.c
@@ -2041,6 +2041,8 @@ consult the installation file that came with this distribution, or visit \n\
 						 */
 						parent = 0;
 
+						php_child_init();
+
 						/* don't catch our signals */
 						sigaction(SIGTERM, &old_term, 0);
 						sigaction(SIGQUIT, &old_quit, 0);
diff --git a/sapi/cli/php_cli_server.c b/sapi/cli/php_cli_server.c
index b7e3e5fa1be2a..92c6ecbdfb581 100644
--- a/sapi/cli/php_cli_server.c
+++ b/sapi/cli/php_cli_server.c
@@ -2530,6 +2530,7 @@ static void php_cli_server_startup_workers(void) {
 #if defined(HAVE_PRCTL) || defined(HAVE_PROCCTL)
 				php_cli_server_worker_install_pdeathsig();
 #endif
+				php_child_init();
 				return;
 			} else {
 				php_cli_server_workers[php_cli_server_worker] = pid;
diff --git a/sapi/fpm/fpm/fpm_php.c b/sapi/fpm/fpm/fpm_php.c
index fb02a3e191775..55b0377c1ad06 100644
--- a/sapi/fpm/fpm/fpm_php.c
+++ b/sapi/fpm/fpm/fpm_php.c
@@ -253,6 +253,9 @@ int fpm_php_init_child(struct fpm_worker_pool_s *wp) /* {{{ */
 		limit_extensions = wp->limit_extensions;
 		wp->limit_extensions = NULL;
 	}
+
+	php_child_init();
+
 	return 0;
 }
 /* }}} */
diff --git a/sapi/litespeed/lsapi_main.c b/sapi/litespeed/lsapi_main.c
index 432c0338c46da..d095f9ae90ec3 100644
--- a/sapi/litespeed/lsapi_main.c
+++ b/sapi/litespeed/lsapi_main.c
@@ -1395,6 +1395,8 @@ void start_children( int children )
             switch( pid ) {
             case 0: /* children process */
 
+                php_child_init();
+
                 /* don't catch our signals */
                 sigaction( SIGTERM, &old_term, 0 );
                 sigaction( SIGQUIT, &old_quit, 0 );
diff --git a/sapi/litespeed/lscriu.c b/sapi/litespeed/lscriu.c
index 09ad53e233c62..042f5fb7a2b5c 100644
--- a/sapi/litespeed/lscriu.c
+++ b/sapi/litespeed/lscriu.c
@@ -85,6 +85,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "lscriu.h"
 
 #include <Zend/zend_portability.h>
+#include "main/php_main.h"
 
 #define  LSCRIU_PATH    256
 
@@ -459,6 +460,7 @@ static void LSCRIU_CloudLinux_Checkpoint(void)
     else {
         s_restored = 1;
         LSAPI_reset_server_state();
+        php_child_init();
         /*
          Here we have restored the php process, so we should to tell (via
          semaphore) mod_lsapi that we are started and ready to receive data.
@@ -532,6 +534,7 @@ static void LSCRIU_try_checkpoint(int *forked_pid)
         LSCRIU_Wait_Dump_Finish_Or_Restored(iPidParent);
         LSCRIU_Restored_Error(0, "Restored!");
         LSAPI_reset_server_state();
+        php_child_init();
         s_restored = 1;
     }
     else {