检测页面是否合法链接过来

<?php
$dbh = @mysql_connect("localhost:3306","root",""); 
if(!$dbh){die("error");} 
mysql_query("set names utf8"); 
@mysql_select_db("music", $dbh);
if(count($_GET)>0)
{
    /*
    * 功能：检测页面是否合法链接过来
    * 如果为非法，给予提示，并关闭窗口.
    */
    $_SESSION['HTTP_REFERER']=isset($_SERVER['HTTP_REFERER'])?$_SERVER['HTTP_REFERER']:"";
    function CheckURL()
    {
        //如果为手动输入的地址，则提示并关闭窗口
        if($_SESSION['HTTP_REFERER']=="")
        {
            echo "<meta http-equiv='Content-Type'' content='text/html; charset=utf-8'>";
            echo "<script>";
            echo "alert('请不要恶意攻击，我们已经追踪到你的ip');";
            echo "window.close();";
            echo "</script>";
            exit;
        }
    }
    CheckURL();
}
?>