Here is a shorter version of Arevos' code. It solves the issue of exploding on "<br />" brought up by DaWei.
<?php
$filename = 'syntax_h.php';
$num_lines = count(file($filename));
?>
<div class="lineNumbers">
<code><?php print implode(range(1, $num_lines), "\n<br />") ?></code>
</div>
<div class="sourceCode">
<?php highlight_file($filename); ?>
</div>