php生成随机数据（数字、大小写字母混合）

<?php
header("content-type:text/html; charset=utf-8");
function random($length, $chars = '0123456789') {
	$hash = '';
	$max = strlen($chars) - 1;
	for($i = 0; $i < $length; $i++) {
		$hash .= $chars[mt_rand(0, $max)];
	}
	return $hash;
}
$length=isset($_GET['length'])?$_GET['length']:'6';
echo "<h1>".random($length, '123456789abcdefghijklmnpqrstuvwxyzABCDEFGHIJKLMNPQRSTUVWXYZ')."</h1>";