li class – can I add php?

I have my posts assigned into an unordered list for my category pages. I want to add a class to each

• based on the post’s taxonomy for the purposes of filtering posts (yes, I know that there are plugins that filter but I want to code it myself).

Can I do something like:
<li class="<?php the_tags() ?>">

Also…if I need to define an element (a la $thetag), can you give me specific directions how to do it? I am learning about java through trial and error and this is one area I am fairly hazy on.

Thanks!