Manual:$wgExtensionFunctions

Details
$wgExtensionFunctions is an array of names of functions without parameters for the setup of extensions. It is defined in DefaultSettings.php without assigning any values. Typically each extension has one setup function associated with it. The array element concerned is typically defined in the extension file itself, with a statement of the form

Each setup function is then called from /includes/Setup.php. It does this by applying to each value in the array. (See php manual for more info.)

For hook type extensions, typical content of this setup function is:

i.e. a call to function setHook in Parser.php, specifying the function defining the HTML to which a text between XML-style tags has to be converted.

In the case of a parser function:

i.e. a call to function setFunctionHook in Parser.php, associating the magic_word_id of the parser function with the name of the PHP function defining it.

In the latter case we also have to add an element to array $wgHooks["LanguageGetMagic"], to associate magic words (function names that can be used in the wikitext) with magic_word_id.