only run frankenphp_register_atfork once, actually showed up in pgo data

Author: henderkes

caddy/frankenphp/default.pgo

@@ -90,6 +90,10 @@ __thread HashTable *sandboxed_env = NULL;
 #ifndef PHP_WIN32
 static bool is_forked_child = false;
 static void frankenphp_fork_child(void) { is_forked_child = true; }
+
+static void frankenphp_register_atfork(void) {
+  pthread_atfork(NULL, NULL, frankenphp_fork_child);
+}
 #endif
 
 void frankenphp_update_local_thread_context(bool is_worker) {
@@ -711,7 +715,9 @@ PHP_FUNCTION(frankenphp_log) {
 PHP_MINIT_FUNCTION(frankenphp) {
   register_frankenphp_symbols(module_number);
 #ifndef PHP_WIN32
-  pthread_atfork(NULL, NULL, frankenphp_fork_child);
+  /* MINIT runs once per ZTS thread — guard the atfork registration */
+  static pthread_once_t atfork_once = PTHREAD_ONCE_INIT;
+  pthread_once(&atfork_once, frankenphp_register_atfork);
 #endif
 
   zend_function *func;