php 防止恶意刷新

<?php 
 session_start(); 
 $k=$_GET['k']; 
 $t=$_GET['t']; 
 //防刷新时间 
 $allowTime = 1800; 
 $ip = get_client_ip(); 
 $allowT = md5($ip.$k.$t); 
 if(!isset($_SESSION[$allowT])){ 
 $refresh = true; 
 $_SESSION[$allowT] = time(); 
 }elseif(time() - $_SESSION[$allowT]>$allowTime){ 
 $refresh = true; 
 $_SESSION[$allowT] = time(); 
 }else{ 
 $refresh = false; 
 } 
 ?>