Az önce yazdım denemiştir. Demo
<!DOCTYPE html>
<html lang="tr">
<head>
 <meta charset="UTF-8" />
 <meta http-equiv="X-UA-Compatible" content="IE=Edge" />
 <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no" />
 <title>Ekran Genişlik Tespiti</title>
</head>
<body>
<p>Ekran Genişlik: <?=$_COOKIE['screen_width'];?></p>

<?php    
    if ($_COOKIE['screen_width'] > 768) 
    {
        echo "Bilgisayar";
    }
    else
    {
        echo "Mobil";        
    }    
?>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script>
<script>
$(document).ready(function () {    
    var width = $(window).outerWidth();
    var cookieName = 'screen_width';
    var width = (+ width);
    var tarih = new Date();
    tarih.setMonth(tarih.getMonth() + 12);
    var alanAdi = window.location.hostname;
    document.cookie = cookieName +"=" + width + "; expires=" + tarih + "; path=/; secure; domain=." + alanAdi;    
});
</script>
</body>
</html>