Create Unique AlphaNumeric

function MakeUnique($length=16) {
           $salt       = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
           $len        = strlen($salt);
           $makepass   = '';
           mt_srand(10000000*(double)microtime());
           for ($i = 0; $i < $length; $i++) {
               $makepass .= $salt[mt_rand(0,$len - 1)];
           }
       return $makepass;
}