多个进程写一个文件

<?php
function writeFile()
{
   $fp = fopen('test.txt', 'w+');
   if (flock($fp, LOCK_EX)) {
      fwrite($fp, 'Write Someting');
      //release file lock
      flock($fp, LOCK_UN);
   } else {
      echo "Error locking file!";
   }
   fclose($fp);
}