Manual:$wgAjaxExportList/en

Details
Registry for functions callable via Ajax. This is a way for extensions (and some functions) to make functions callable from JavaScript. Making an Ajax request to  with the   parameters will result in a   call in PHP, as long as funcname is listed in. Such functions then usually return an AjaxResponse object.

$1 and $2 cause $3 and $4 to be added to this array, respectively.