Manual:$wgCacheEpoch

Details
Set this to current time to invalidate all prior cached pages. Affects both client- and server-side caching.

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: date +%Y%m%d%H%M%S

To force a file, such as LocalSettings.php to be re-read when touched, you can use this code (taken from LocalSettings.php, MW < 1.17). $wgCacheEpoch = max( $wgCacheEpoch, gmdate( 'YmdHis', @filemtime( __FILE__ ) ) );
 * 1) When you make changes to this configuration file, this will make
 * 2) sure that cached pages are cleared.

Since version 1.17, you can use $wgInvalidateCacheOnLocalSettingsChange to automatically invalidate cache whenever LocalSettings.php is modified. In earlier versions, code in LocalSettings.php did a similar thing.