function getRandomString($len, $chars=null)
{
??? if (is_null($chars)){
??????? $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
??? }?
??? mt_srand(10000000*(double)microtime());
??? for ($i = 0, $str = '', $lc = strlen($chars)-1; $i < $len; $i++){
??????? $str .= $chars[mt_rand(0, $lc)];?
??? }
??? return $str;
}

PHP教程
echo getRandomString(2);

PHP教程
echo chr(mt_rand(65, 90);

PHP教程
echo chr(mt_rand(97, 122));