Be sure to have a full backup of the forum database before the php upgrade takes place, in case the upgrade causes database corruption.
The forum footer says it's currently running on "vBulletin® Version 3.8.4".
https://forum.vbulletin.com/forum/vb...n-for-vb-3-8-4
says
"
vBulletin 3.8.4 and lower will not work with PHP 5.3.0 or higher."
If that is true, vBulletin needs to be upgraded before the forced php upgrade takes place, otherwise it might break completely and not work at all anymore.