Unexpexted shell command result

switch($modx->event->name)
    {
      case 'OnBeforeSnipFormSave':
      $s = $modx->event->params;
      $o = $s['id'];
      $object = $modx->getObject('modSnippet',$o);
      $content = $object->get('snippet');
      $file = fopen('check.php','a+');
      fputs($file, $content);
      $exec = 'php -l check.php';
      $modx->event->output(shell_exec($exec));
          break;
    }