Manual:$wgServerName/en

''This variable was formerly available from 1.3.8 and then removed in 1.17.0 (r73950). It was re-introduced in 1.24.0.''

Details
The name of the server without the protocol or trailing slash. (eg, "en.wikipedia.org"). It is automatically calculated based on. It should not be customized, and will generate a warning if set.

Used in:
 * ( property)
 * Default value of server for and  emails.