I had trouble finding the implementation of CURRENTMONTH in includes/parser.

It turns out it's implemented by Parser::getVariableValue in lowercase

switch ( $index ) {
         case 'currentmonth':
              ... do something

even though {{currentmonth}} in wikitext doesn't work. The only doc for the $index parameter to this function is @param int $index , but it's definitely not an integer.

A quick fix is to change the doxygen comment to something like

@param string $index   Magic variable identifier. Note that it's lowercase by the time this function is called.

I think MagicWord.php converts from capitalized magic variables to lowercase internals, I'm not sure where this should be documented.