php timestamp based authentication

<?php

    const salt = "such_salt";
    const timeframe = 10;

    function gen()
    {
        return strtoupper(substr(md5(salt.time()),0,12));
    }

    function auth()
    {
        $valid = false;

        if (timeframe<1)
        {
            $len_t = 1;
        }
        else
        {
            $len_t = round(timeframe);
        }

        if (isset($_GET['pass']))
        {

            $t = time();

            for ($i=0; $i<$len_t; $i++)
            {
                if (strtoupper(substr(md5(salt.($t-$i)),0,12)) == $_GET['pass'] )
                {
                    $valid = true;
                    break;
                }
            }

        }

        return $valid;
    }

    echo "<pre>";

    echo "<a href=\"?pass=".gen()."\">auth</a><p>";

    var_dump(auth());

    echo "</pre>";

?>