You want to know about codepad. Codepad is a simple-to-understand Web site that accepts free-form text input, interprets your text as the source of a program in C, Lua, or a dozen other languages, and returns the result to you.
What's the point of that? Experience it for yourself to find out. Suppose you have a question about the semantics of a particular PHP function; with a good IP connection, it should be quick work to look through the extensive on-line PHP documentation, or ask the IRC phpfreaks, or do a general search, or even quickly download the latest PHP installation. Those are all sensible approaches, each with an advantage in certain situations. codepad is at least as quick as any of the alternatives, and all its "artifacts"--what source you're running, the output that results, and your comments on the two--appear at URLs that are easy to share. codepad is simply a great resource for talking over coding problems.
There are limits to codepad: it only runs one specific version of each language processor, security considerations entail that each language is a proper subset of what is generally available on the desktop (codepad doesn't allow branch-to-reboot, for example), it requires a live IP connection, it probably doesn't build in the particular language extension that happens to interest you, and so on. Within its domain, though, it's just delightful to be able to take advantage of codepad.