This is a prototype of 'Everyone's Mouse Pointer' proposed by not_only_but_also.
Being able to see the cursor of everyone who's looking at a page would make the internet far less solitary.
Unfortunately, HTTP wasn't really designed for this kind of thing, and the result is quite laggy.
A dedicated server (rather than just a PHP script) would be much faster.
A bigger problem, if this were to be integrated into a popular web page, is that everyone has different screen resolutions and font sizes.
The pointer coordinates are currently just in pixels, a percentage of the screen size might be more effective.
There's no real security either, nothing to stop someone spoofing hundreds of cursors at once.
I should probably fill this page with some interesting things to point your mouse at...