Manual:$wgServerName

This feature was removed completely in version 1.17.

General Settings: $wgServerName
The name of the server without the protocol.
Introduced in version:	1.3.8
Removed in version:	1.17 (r73950)
Allowed values:	(string)
Default value:	(dynamically created)

Details [edit]

The name of the server without the protocol or trailing slash. (eg, "en.wikipedia.org"). See $wgServer for details about how it is calculated.

The syntax to get a website to answer on an IP is:

$wgServerName = "foo.bar.com";
$wgServer = "{$wgProto}://{$wgServerName}";