Well, if you want to do it yourself you need to learn how to program. Then start programming. Maybe you'll complete it, or maybe you'll get stuck on something we can help with. Until then, have fun.
But you won't be able to do it in PHP, because PHP doesn't know what a mouse is. You'll need the program to run in the browser, so you'll need to use something like JavaScript, Flash, or Java (see, for example how pronto1.alphatrust.com do it, for example); a solution that uses the HTML 5 draft's <canvas> element would restrict access to browsers that support it (I think those would be FireFox, Safari, Opera, and Chrome, and I understand that there are plugins and JavaScript emulations for IE support). But generally it would need to be something something that can respond to mouse activity. And that means something other than PHP.
....does that count as a silly answer?
