How can I make visual studio highlight .php files
Tools -> Options -> Text Editor -> File Extension
in the Extension box, put .php. For the Editor, you can select HTML Editor. Click Add, then OK
You can also install the PHP Tools for Visual Studio add-in. Tools > Extension and Updates and then search for PHP online. Your first result should be this add-in.
If not, you can get it from here:
http://visualstudiogallery.msdn.microsoft.com/6eb51f05-ef01-4513-ac83-4c5f50c95fb5
If you want intellisense, debug, and other advanced functionality, you could get the php ide for VS
If all you need is syntax highlighting, check out this link. It's what I'm currently using, but doesn't have all the goodies of vs.php.