Manual:$wgExtensionDirectory

Server URLs and file paths: $wgExtensionDirectory
The file system path to the extensions directory.
Introduced in version:	1.25.0 (Gerrit change 209216; git #cf2678e2)
Removed in version:	Still in use
Allowed values:	(relative path)
Default value:	`null` (1.38+) "{$IP }/extensions" (1.25-1.37)
Other settings: Alphabetical \| By function

Details

The file system path to the extensions directory (relative path). This controls where MediaWiki will look for extensions on the server itself.

This is set to "{$IP}/extensions" by Setup.php before loading local settings.

Ignored when the exact path to a particular template's .json configuration file is given to the wfLoadTemplate() function.

Example: wfLoadTemplate( 'TemplateName', '/path/to/TemplateName/template.json' );

$IP
$wgExtensionAssetsPath – the URL for extensions
$wgStyleDirectory – filesystem path for skins
$wgStylePath – the URL for skins.
Manual:Extensions - extensions let you customize how MediaWiki looks and works.
Manual:Extension registration - instructions for both building and loading skins and extensions.