From a brower's perspective, PHP is no different than HTML. A browser would be anything that gets the page from the server, be it Internet Explorer, Netscape, Lynx, or any search engine.
Basically, your web server is never going to spit out PHP code, it will always interperet it first. If you look at the headers that it sends with a .php file, it will most likely (if it's setup properly) be sending "Content-type: text/html". You can easily setup your server to run PHP on even .html files, so you could put PHP code into .html files and it would be executed.
The only things you have to keep in mind when designing a php site for search engines is:
1) search engines won't submit forms, they will only follow links (<a href...>).
2) search engines don't support cookies, so that could mess up your site if it relies on sessions. Consider using some of the meta tags that will prevent that page from being indexed for pages that require sessions (shoppping carts, etc)
3) if you pass a session ID in the url, search engines will remember and index that session ID (since it is part of the url). That means that anyone coming to your site from that search engine will be using the same session ID, which is bad (and totally defeats the purpose of session IDs).
4) some search engines will consider only the filename, so if you're using a scheme like: www.domain.com/main.php?page=Products, then that might not be considered a different page from www.domain.com/main.php?page=Services, since they're both "main.php"
As long as you plan your site properly, none of these will be a problem. Just make sure to test it in a plain old text-only browser that doesn't support cookies. You can even go as far as detecting the $HTTP_USER_AGENT and if it's not a recognized browser, fall back to a default support that doesn't require anything mentioned above.
