Manual:$wgScriptExtension/en

Details
Extension to append to script names by default. This can either be ".php" or ".php5".

Some hosting providers used PHP 4 for *.php files, and PHP 5 for *.php5. This variable was provided to support those providers.

Now that this setting has been removed you can set up URL rewriting from ".php5" files to ".php" files to continue supporting ".php5" entry points.