unordered list

I'm looking to do a site re-design right now, and I want to have the ad network ads in an unordered list, which isn't really obvious for me...

So I thought about changing the line in the ad_network_298.php file from:

$ad_network = implode ('</li><li>', $ad_network);

PHP:

to

		$ad_network = '<li>'.$ad_network[0].'</li><li>'.$ad_network[1].'</li><li>'.$ad_network[2].'</li><li>'.$ad_network[3].'</li><li>'.$ad_network[4].'</li>';

PHP:

And then I can have in my page files, something along the lines of:

<ul class="ad_network">
<? 
include('/usr/local/htdocs/ad_network_298.php');
echo $ad_network;
?>
</ul>
It works, but I'm wondering if it's OK to do something along these lines - in order to have more control over the visual formatting, or will it affect the validation.

PHP: