<div class="highlight php"> ... </div>
Where 'php' could be any language supported by GeSHi.
Note, you'll also need to grab GeSHi (version 1.0.8.9 tested) from here and unpack the archive into the plugin's directory (ie: [plugin directory]/geshi/geshi.php should exist).
It's recommended that you use CDATA blocks (which the plugin will automatically strip), just in case the plugin is ever disabled:
<div class="highlight php">
<![CDATA[
<?php
$foo = 'bar';
?>
]]>
</div>