I think it depends on the search engine and how the script using runs.

For normal PHP stuff, search engines do fine. I could have an entire site done in PHP with php extensions, and a search engine would crawl it all.

Things may have changed now, but I know for a while not all search engines would crawl an entire site if you had variables in your url (http://www.mydomain.com/index.php?go=true ). Session IDs in URLs are also a problem sometimes.

Things may have changed though. I'm not up to date on all the search engine tricks. Someone else might be able to tell you better.

But anyway, with that said... You should be able to optimze PHP pages just like you would HTML pages. That's because only HTML is displayed to visitors, the PHP is processed on the server. You just want to make sure that your PHP scripts are generating the optimized HTML.