最简单和可移植的事情是在一些预定义的文件上使用文件锁。
例如：

<?php
  $flockfile = fopen('lockfile.lck', 'c'); // Open a file, the file will be closed when the script ends
  if (!flock($flockfile, LOCK_EX | LOCK_NB)) {
    // If cannot lock the file - just exit
    fclose($flockfile);
    die("Another process is running");
  }

下面是一个使用提供咨询锁的数据库的解决方案，尽管在数据库中存储令牌也可以完成类似的工作。

1. The script checks if an advisory lock is present in the database, and if is then exits.
2. If the lock is not present then the script places an advisory lock for transaction.
3. The lock is automatically released when transaction commits or script crashes.