[Dave]

Upgrade your PHP version to at least 5.5.

Originally Posted by vBulletin.com

vBulletin 4.2.5 requires PHP 5.6.0+ and MySQL 5.5.8+.
vBulletin 4.2.4 requires PHP 5.5.0+ and MySQL 5.0.0+.
vBulletin 4.2.3 requires PHP 5.4.0+ and MySQL 4.1.0+.
vBulletin 4.2.2 requires PHP 5.2.0+ and MySQL 4.1.0+.

Also if you're constantly being hacked then an upgrade may not necessarily fix that, it's entirely possible that the hacker uploaded a backdoor on your server in order to gain access anytime the hacker desires.