The folks working on the official PHP Documentation have done a marvellous job in providing a mostly easy to follow documentation, with good examples. However, as with anything, there can still be some improvement. I see two shortcomings and these have to do with structure rather than content, and I think the community can help solve them:
1) Access for the average time-pressed developer, to the insights (and potentially corrections, readability improvements, etc. as well) added in the user-contributed comments would be much more facilitated if they could be integrated into a community wiki.
2) Along these same lines, making the content fully open (as in GNU Free Documentation License), despite the already somewhat liberal copyright policy of the existing documentation, would welcome more contributions.
Mozilla has enabled such a strategy in publishing its "Creating Applications with Mozilla." Why not make an open print copy of the results after it is "done" (or done well enough)?
So, with this in mind, a page at Wikibooks was created ( an earlier creation exists but with an apparently less comprehensive, though of course still potentially useful purpose). Since it is under GNU, PHP.net or Zend.com could, if desired, port over its pages if a significant body of pages were created.
And, although of a different nature, a few other pages have been set up to encourage community documentation efforts to give greater access to "hacking" the popular open-source Mediawiki, phpBB, etc. for customization, extensibility, and ideally even integration of these great programs together. Contributions are most needed and welcome!