message: Serialize can only be used on PHP7.
diff: @@ -422,8 +422,10 @@ void php_swoole_onBufferFull(swServer *, swDataHead *);
  void php_swoole_onBufferEmpty(swServer *, swDataHead *);
  int php_swoole_length_func(swProtocol *protocol, swConnection *conn, char *data, uint32_t length);
  +#if PHP_MAJOR_VERSION >= 7
  PHPAPI zend_string* php_swoole_serialize(zval *zvalue);
  PHPAPI int php_swoole_unserialize(void *buffer, size_t len, zval *return_value, zval *object_args);
  +#endif
  static sw_inline zval* php_swoole_server_get_callback(swServer *serv, int server_fd, int event_type)
  {
  