<?php
$num='0123456789';
$let='abcdefghijklmnopqrstuvwxyz';
$LET='ABCDEFGHIJKLMNOPQRSTUVWXYZ';
$str=$num.$let.$LET;//从该给定的字符串中随机抽取元素组成定长的字符串

$len = strlen($str);

$string = '';
for($i = 1; $i <= 4; $i++){
   $string .= substr($str,mt_rand(0,$len-1),1);
}
echo $string;

?>