how to PHP if condition with browser width?

I have some script like below but it does not work

<?php
$width = "<script>document.write(window.outerWidth);</script>";
$width2 = (int)$width;
echo $width2;
if ( $width2 < 800 ) { ?>
    bad
<? } else { ?>
    ok
<? } ?>

please help me ...