Help Needed w/ PHP

I'm trying to help out a friend that has been stuck with a site built with a proprietary CMS.

In an effort to help her site's SEO, I've created a few hacks that should improve things, but my overall PHP-fu is weak.

I'm trying to hack the Description meta tag so that it will use a truncated portion of a variable (please excuse me if my nomenclature is incorrect) if that variable exists. If the variable does not exist, an alternate "standard" piece of information should be used.