We have received this warning on a number of sites, and have gone in and upgraded PHP on them.
Upon reconnecting to WordPress dashboard, the PHP warning persists.
I've tried deleting and reinstalling Akeeba BFW 2.4.2 via FTP and the problem goes away.
Then, after a delay of a day, I log into the WordPress dashboard and the warning is back. Meantime, the version of Akeeba 2.4.2 installed is working - just the warning pane on the plug-in page persists.
Also tried refreshing cache on plugin page multiple times.