The code below will allow you to detect screen resolutions and take action. The code works by using JavaScript and making available to PHP by way of cookies.

<?
/**************************************************************************************
Originally from www.phpbuddy.com
Author: Ranjit Kumar (Cheif Editor phpbuddy.com)
Modified by: Ying Zhang (Dodo) pure-essence.net main modifications:
1. To check currently saved screen resolution cookie
2. To allow inclusion by other php files

In order to use this file in another php file, use:

$callget_res_page_name = $REQUEST_URI;
$GLOBALS[callget_res_page_name] = $callget_res_page_name;
include("get_resolution.php");
echo $screen_width." is your screen width and ".$screen_height." is your screen height.";

*****************************************************************************************/
?>

<script language="javascript">
<!--
function writeCookie()
{
var today = new Date();
var the_date = new Date("December 31, 2023");
var the_cookie_date = the_date.toGMTString();
var the_cookie = "users_res="+ screen.width +"x"+ screen.height;
var the_cookie = the_cookie + ";expires=" + the_cookie_date;
document.cookie=the_cookie
if (document.cookie){
        location = '<?=$GLOBALS[callget_res_page_name]?>';
}

}

function checkRes(width, height) {
        if(width != screen.width || height != screen.height) {
                writeCookie();
        } else {
                return true;
        }
}
//-->
</script>

<?
If(isset($HTTP_COOKIE_VARS["users_res"])) {
        $screen_res = $HTTP_COOKIE_VARS["users_res"];
        $screen_res_tmp = explode("x", $screen_res);
        $screen_width = $screen_res_tmp[0];
        $screen_height = $screen_res_tmp[1];
        ?>
        <script language="javascript">
        <!--
        checkRes(<?=$screen_width?>, <?=$screen_height?>);
        //-->
        </script>
        <?
}
else //means cookie is not found set it using Javascript
{
?>
<script language="javascript">
<!--
writeCookie();
//-->
</script>
<?
}
?>