What about PHP4?

PHP4 is over 5 years old. While there is nothing in Phorum (as of 5.1) that is known not to work with PHP 4.3.0 and up, the Phorum team does not currently test any Phorum code against PHP4. Therefore, you use Phorum with PHP4 at your own risk. We will not shun you in the support forums for using it. But, if your problem is because of PHP4, there will likely be no remedy.