The idea is to process the php file (and yes, it's still saved as *.php) and send output (be it html, javascript, css, image whatever) to the browser. Now JavaScript has to be supported by browser not the server.
As to the Help when over a picture: if it's short line of text then put it in title="your help text" inside <img> tag.
<img src="whatever.gif" title="Help text">
If it is supposed to show somewhere on the page - JavaScript is the way to go.