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But there's no command to get an explaination. The documentation is not included in the php code.
If it's a function of the php core or an extension take a look at the online manual, http://www.php.net/manual/
If they are functions/methods you built, at the time of building it is a good idea to document/annotate the function. A useful tool for documentation is PhpDoc which can help build a "website" of documentation.