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 the following line, whose timestamp is generated via :

If you are using a Unix-like system 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):