Javascript / Php

<?php function setscreen() { $height = "<script>document.write(screen.width);</script>"; $width = "<script>document.write(screen.height);</script>"; if (($height == "640") && ($width == "480")) { echo "height == 640 & width = 480"; } elseif (($height == "800") && ($width == "480")) { echo "2"; } elseif (($height == "800") && ($width == "600")) { echo "3"; } elseif (($height == "1024") && ($width == "576")) { echo "4"; } elseif (($height == "1024") && ($width == "768")) { echo "Height = 1024 & width = 768"; } echo "Height == $height & width == $width"; } ?> <?php setscreen(); ?> gives me this Height == 1024 & width == 768 but the html for that is Height == <script>document.write(screen.width);</script> & width == <script>document.write(screen.height);</script> and i wanne work with the 1024 in php and not with the html ,, what can i do to run the javascript at server side and not on the client side