Perhaps I am misinterpreting your statement, but it almost sounds like you blame PHP for the lack of documentation of those libraries. Really you should be blaming the authors of the libraries for poorly documenting them, or the developer who chose those libraries, for choosing ones with poor documentation.
I much prefer Ruby's documentation to PHP's. Especially when it comes to 3rd party code, I don't have one gem in my current Rails project that isn't very nicely documented. Compare that to the PHP I work on during the day and almost all the 3rd party libs have no documentation.