Naturally, you'd have to make it decentralized and publically editable, i.e. anyone can edit whatever parts of whatever public instances (saving version history, of course), and anyone can mirror (whatever version of) whatever parts they choose (possibly for public display), which can get, send, updates about the mirrored parts.
Thus there is no censorship, yet there is all the censorship anyone could want. And there is robustness (of the popular data, at least). (To solve the problem of saving unpopular data, one would need to integrate payments into the system, e.g. for each plank-time you host my data, I pay you m money, for each fetch, n money,,)
Perhaps PHP is not the best language for such a project. (My predjudice is to C/C++, or literate programing.) Perhaps focus on composing, then writing, a solid server in a real language, with an elegant interface, and a reference client. Perhaps extensible to other protocols, similar interfaces,,