Anyone know of an API (php preferable but I\'d be interested in any language) for creating wiki-like data storage?
How about any resources on rolling your own plaintext
Will's huge answer is right on, but can be summed up, I think: you need to store the versions, and then you need to store the metadata (who what when of the data).
But your question was about resources on Wiki-like versioning. I have none (well, one: Will's answer above). However, about the storage of Wikis, I have one. Check out the comparison matrix from DokuWiki. I know. You're thinking "what do I care what brand of DB different Wikis use?" Because DokuWiki uses plain text files. You can open them and they are indeed plain. So that's one approach, and they've got some interesting arguments as to why DBMS are not the best way to go. They don't even hold much metadata: most of the stuff is done through the flat files themselves.
The point of the DokuWiki for you is that maybe it's a relatively simple problem (depending on how well you want to solve it :)