Manual:$wgSpecialPages/en

Details
$wgSpecialPages stores a list of custom special pages defined for a MediaWiki installation. The array is keyed by a non-localized id for the special page. The value assigned to each key is an specifier (MediaWiki 1.34+), or one of the following:


 * string: a string containing the name of a subclass of.
 * array: an array where the first element is the name of the subclass and all remaining elements are parameters that should be passed to the constructor of that subclass. (This format was deprecated in MW 1.18 and is no longer recognized as of MW 1.32.)
 * callback: since MW 1.24,  expected to return a  instance. Useful for full control over the special page's construction parameters.
 * an actual SpecialPage object.

For more information, please see SpecialPage.php