Detecting browser

//DETECTING BROWSER PHP
*/
/*$_SESSION["counter"]++;
if($_SESSION["counter"]===7){
    header('Location: /l4/gratulacje.html');
}

*/
if(strstr($_SERVER['HTTP_USER_AGENT'],"Mozilla  ")){
    header('HTTP/1.x 404 Not Found');
    exit("Not Found");
}
else if(strstr($_SERVER['HTTP_USER_AGENT'],"Android")){
    header('Location: /l4/mobile.html');
}

//DETECTING BROWSER JS
function Browser()
    {
    if((navigator.userAgent.indexOf("MSIE") != -1 )
      alert('Remember to activate your copy of Windows');
    }
    else
    {
       alert('Take it easy. Keep suring');
    }
    }
    window.onload=Browser();
</script>