php如何解决多线程同时读写一个文件的问题

<?php
header("content-type:text/html;charset=utf-8");

  $fp = fopen("lock.txt","w+"); 

    if(flock($fp, LOCK_EX)){// 进行排它型锁定
        fwrite($fp,"Write something here\n");
        flock($fp, LOCK_UN);// 释放锁定
    }else{
        echo "Couldn't lock the file !";
    } 

    fclose($fp);
?>

source:http://www.bcty365.com/content-10-2954-1.html