Ask HN: Any exemplary PHP codebases out there?

I've got my CS degree, I'm a competent programmer, and I know the principles behind the web, but I'm a bit short on experience with big projects and major industry-standard languages. In particular, web development seems like the sort of thing I could do well -- but almost everyone wants PHP, which I've only ever dabbled in. So I'm thinking a good place to start would be looking at (and maybe contributing to) some existing big PHP projects; ones that are considered shining examples of how to "do PHP right". Particularly considering PHP's reputation for being awfully easy to "do wrong". Can anyone recommend any such projects?