As with the Redis cache store, igbinary is only supported when the PHP igbinary module has been installed and enabled... https://www.php.net/manual/en/book.igbinary.php After ensuring the above is available, on a fresh Moodle install... Go to Site administration -> Plugins -> Caching -> Configuration Add a new File cache store instance On the "Add File cache store" settings page for this new instance, if the igbinary module is available, the "User serializer" select box will have two available options... "The default PHP serializer." and "The igbinary serializer." Select "The igbinary serializer." and click the "Save changes" button. Make the new file cache store instance the default application and session stores. Create some new courses. Confirm everything should work as expected with the igbinary option selected. Also new unit tests should also cover getting and setting cache items with both the PHP serializer and igbinary.