[php]<?php
$seed = "onlyonce";
$chk=$_COOKIE["once"];
echo $chk;
if ($chk = $seed){
$an = "请勿刷新";
} else {
$an = "谢谢访问";
}
setcookie("once", $seed,time()+3);
echo $an;
?>[/php]

我想设置三秒内不准刷新

请问这段要怎么写?