You shouldn't receive the PHP file. PHP is a scripting language that allows you to make web pages with dynamic content. The web-server that sent you the file is supposed to be configured to read in the PHP file, run the instructions inside it, and return to you HTML (a web page) or other data.
You can, of course, place the file in your Sites directory and then enable web-sharing in the system preferences (under Sharing). Then, you ought to be able to see the file at
http://localhost/~name/loademailkit.php (replace 'name' with your short user name, and make sure that you changed the PHP file to be all lower case). HOWEVER, there's the problem that the PHP script will generally reference resources on the server where it originated, so it may not do anything at all.
If you'd simply like to see the PHP programming code, start up your favorite text editor and drag the PHP file's icon onto the icon for the text editor.