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Looks like php_user_filter is really used out there, just encountered a stackoverflow question about it.
There are some pages already undocumented for a long time...
I searched PHP source code and put the non-explained parts together in an answer to that question: http://stackoverflow.com/a/31132646/2153758
Might be eventually helpful to the one who will complete these missing docs.
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bwoebi has been bugging me about this, so I feel it only right to own this task. :)