产生随机密码的函数：
调用：
１． echo random(6,0);
　产生６位的包括大小写字母数字的密码
２． echo random(15);
　产生１２位的只包括数字的密码

function random($length,$ctype=2) {
$hash = '';
$chars= '';
if($ctype==0)
$chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz';
if($ctype==1)
$chars = '0123456789abcdefghijklmnopqrstuvwxyz';
else
$chars = '0123456789';
$max = strlen($chars) - 1;
mt_sran