Do something if the visitor is using Internet Explorer.
<?php
$IE6 = ( preg_match( 'MSIE 6', $_SERVER['HTTP_USER_AGENT'] ) ) ? true : false;
$IE7 = ( preg_match( 'MSIE 7', $_SERVER['HTTP_USER_AGENT'] ) ) ? true : false;
$IE8 = ( preg_match( 'MSIE 8', $_SERVER['HTTP_USER_AGENT'] ) ) ? true : false;
if ( ($IE6 == 1) || ($IE7 == 1) || ($IE8 == 1) ) { ?>
<p>You are using IE 8 or lower</p>
<?php } else { ?>
<p>You are using a modern browser</p>
<?php } ?>
WordPress Snippet