PHP程序如何防止站外提交数据

<?php
$servername=$HTTP_SERVER_VARS[&#39;SERVER_NAME&#39;];
$sub_from=$HTTP_SERVER_VARS["HTTP_REFERER"];
$sub_len=strlen($servername);
$checkfrom=substr($sub_from,7,$sub_len);
if($checkfrom!=$servername){
echo("数据来源有误！请从本站提交！");
exit;
?>

非常有用