PHP websites testing is similar to other web application testing.
In PHP there will be more JS and HTML code used.
So you can check the browser compatibility using http://browsershots.org
Since th ehtml tags and JS will be different in each browser.
So in general I have a question about PHP testing, is there anything special about PHP testing. We have something called PHPUnit which is the frame work for unit testing of PHP. We can use that for testig PHP and then test the compatibility with different browsers