php中使用随机数

<?php
session_start();
$s = array("a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","0","1","2","3","4","5","6","7","8","9");
$r="";

for($i=1;$i<=4;$i++){
$r .= $s[rand(0,count($s)-1)];
}

不要小看  要是叫你写你还写不出来的