iPhone4 retina display detection with PHP and/or JavaScript

I'm creating a detection script that sniffs out any device (currently only iPhone4) with a retina display (or similar) when a user arrives at my site. Because the resolution is greater, I need to push higher res images/graphics. The only solution that I can find (using PHP and JavaScript) is to detect the devicePixelRatio and set a cookie. Here is the code that I am using:

<?php
    $imgPath = "images/";
    if(isset($_COOKIE["imgRes"])){
        $imgRes = $_COOKIE["imgRes"];
        if( $imgRes  >= 2 ){
            $imgPath = "images/highRes/";
        }
    } else {
?>
    <script language="javascript">
        var the_cookie = "imgRes="+window.devicePixelRatio+";"+the_cookie;
        document.cookie = the_cookie;
        location = '<?=$_SERVER['PHP_SELF']?>';
    </script>
<?php
    }
?>

Has anyone come across a better method of doing this or have any suggestions of improving this script. This script does work, it just feels dirty.