Manual:$wgExtensionCredits

Details
An array of extension types and inside that their names, versions, authors, URLs, descriptions and pointers to localized description messages. Note that the version, url, description and descriptionmsg keys can be omitted.

This credit information gets added to the wiki's Special:Version page, allowing users to see which extensions are installed, and to find more information about them.

This is not a configuration setting mentioned in LocalSettings.php. Typically, extension developers will write code that appends to the array contained in this global variable.

See Manual:Developing extensions

Usage
Extension developers can append to the array with the following code:

The description and author fields get parsed as wiki syntax. This is often used to provide links to the author's homepage in the author field. In version 1.12 and newer, descriptionmsg will override description.

type
$type must be one of the following:
 * — API extensions
 * — antispam extensions
 * (added in 1.21) — Wikidata related extensions
 * (added in 1.11) — media handlers
 * — extensions that modify, add or replace functionality in the MediaWiki parser
 * — extensions that modify skins
 * — extensions that add special pages
 * (added in 1.6) — make a new variable
 * — does something else

descriptionmsg
The value of descriptionmsg will be treated as the message key for the extension description: see Manual:Developing extensions. As a standard, it's in the form  (all lower case), or the prefix used for the message keys of the extension, followed by the   suffix.

The description:
 * Must be a single brief sentence: it has no closing period and implies "this extension" as a subject when needed.
 * Is usually in one of the following forms:
 * "Does"/"Adds"/"Allows to do" something (e.g. " Adds tag for poem formatting ");
 * "Allow to do"/"Do" something, with the wiki or the users as subject (e.g. "Hide individual revisions from all users for legal reasons, etc.");
 * "Special page to do" something, or other noun-sentence to describe what the extension is composed of (e.g. "Special page to view global file usage", "Wikimedia specific messages").
 * Includes a link to the (main) added special page(s) if any, using "special page" or "do something" etc. as link labels.

author
Since (mw 1.18?), the author list can contain the special hackish '...' to denote other authors. The triple dot will make MediaWiki insert the version-poweredby-others message.

Example: