There's none. There's imagegrabwindow(), but no way it'll work on a shared host. Maybe you're lucky and find an external .exe or something that you can call using exec(). Other than that, try using a third party service. There are a lot of sites that have some APIs you can use (probably not for free, though).
Good luck with that one, I havn't seen one solution that'll work on a shared host yet - and I've tried many times in the past.
Isn't that on free shared linux host ? siteshot.freehostia.com/?x=800&y=800&url=http://forums.digitalpoint.com/
Oh, and another note, the imagegrabwindow can only be used on a Windows host, so that eliminates ALOT of your shared hosting companies out there.
Not only that, it also requires "Interact with the desktop" and IE to be installed. Good luck finding a host that supports all that.