function makecode($num=4) {
$re = ”;
$s = ‘abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
while(strlen($re)<$num) {
$re .= $s[rand(0, strlen($s)-1)]; //从$s中随机产生一个字符
}
return $re;
}