Thorough code review to ensure we didn't miss anything from the spec

[mousetraps]

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The PHP spec was a bit open to interpretation at times, so it'll be good to have a second pass, and should be a good opportunity to add more tests.

While we're doing this pass, we should submit any remaining PRs to the PHP language spec based on our findings from this project. Already submitted a few PRs, and I've been meaning to submit some more.