php安全开发 添加随机字符串验证，防止伪造跨站

<?php   
class Crumb { 
    CONST SALT = "your-secret-salt";                                                            
    static $ttl = 7200;                                                                                           
    static public function challenge($data) {   
        return hash_hmac('md5', $data, self::SALT);   
    }                                                                                                                
    static public function issueCrumb($uid, $action = -1) {   
        $i = ceil(time() / self::$ttl);   
        return substr(self::challenge($i . $action . $uid), -12, 10);   
    }                                                                                                                
    static public function verifyCrumb($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;                                                                                       
        return false;   
    }                                                                                                              
} 

<form method="post" action="demo.php">   
 <input type="hidden" name="crumb" value="<?php echo Crumb::issueCrumb($uid)?>">   
 <input type="text" name="content">   
 <input type="submit">   
 </form> 

<?php   
if(Crumb::verifyCrumb($uid, $_POST['crumb'])) {   
    //按照正常流程处理表单   
} else {   
    //crumb校验失败，错误提示流程   
}