Why do you want to open it in a browser? PHP needs to be executed on a server.
Anyway: Right click on the document. If there's no item called "Open with" then try holding these keys: Shift, Ctrl, Alt and Cmd. I don't remember which one is the beste choice, but you can see some items in the menu changing. You can then access "Open with".
Or, you open the browser and choose the file via the global menu "File".
If you want to run PHP files on your own computer, I suggest you install MAMP.
To run PHP files, they have to actually be on a server with PHP installed. MAMP basically installs a server on your computer with PHP, so that you can put your PHP files in the MAMP directory and you will be able to run them in your browser. :)