Merge pull request #22416 from stanley-cheung/php-add-fork-flag

PHP: Add allow fork flag to config.m4

src/php/ext/grpc/config.m4

@@ -86,7 +86,7 @@ if test "$PHP_GRPC" != "no"; then
 
   PHP_NEW_EXTENSION(grpc, byte_buffer.c call.c call_credentials.c channel.c \
     channel_credentials.c completion_queue.c timeval.c server.c \
-    server_credentials.c php_grpc.c, $ext_shared, , -Wall -Werror -std=c11)
+    server_credentials.c php_grpc.c, $ext_shared, , -Wall -Werror -std=c11 -DGRPC_POSIX_FORK_ALLOW_PTHREAD_ATFORK=1)
 fi
 
 if test "$PHP_COVERAGE" = "yes"; then