Submitting form data to your website creates security problems

<?php
class Crumb
{
    const SALT = 'http:test.com';
    static $ttl = 7200;
    static public function issueCrumb($uid, $ttl=7200, $action= -1)
    {
        if(intval($ttl)>7200) self::$ttl = $ttl;
        $i = ceil(time() / self::$ttl);

        return substr(self::challenge($i . $action . $uid), -12, 10); 
    }

    static public function challenge($data)
    {
        return hash_hmac('md5',$data, self::SALT);
    }

    static public function verifyCrumb($uid,$crumb, $action= -1)
    {
        $i = ceil(time() / self::$ttl);
        if(substr(self::challenge($i . $action . $uid), -12, 10) == $crumb || substr(self::challenge(($i-1) . $action . $uid), -12, 10) == $crumb){
            return true;
        }else{
            return false;
        }
    
    }
}
$crumb = Crumb::issueCrumb(1);

//var_dump($crumb);
//die;
$crumb = 'fc3c0f7c23';
$ver = Crumb::verifyCrumb(1,$crumb);
var_dump($ver);