Manual:$wgCacheEpoch

Details
Set this to current time to invalidate all prior cached pages. Affects both client- and server-side caching, including assets controlled by the ResourceLoader.

For a quick copy-paste you can use :

If you are using Linux you can get the current date by entering the following on the command-line:

To force a file, such as "LocalSettings.php" to be re-read when touched, you can use the following code (MW < 1.17):

Since version 1.17 of MediaWiki, you should use  to automatically invalidate cache whenever "LocalSettings.php" is modified.