PHP is a server side language, which means that all of the PHP is executed on the server, so the browser on your computer never even sees it. What your browser gets is the result of the PHP, but not the PHP itself. This means your browser doesn't need any special settings to see PHP pages, enable PHP code, or anything else.
If you are having trouble with a PHP page, it is probably because there is something wrong with the code, or the HTML encasing it. But one thing is for sure, it is not a browser compatibility issue!