These versions of UBB.Threads weren't meant to be run on PHP5, they were written in a time of PHP4.
I just dug through the archive.org and it looks like UBB.Threads v6 had the system requirements listed as:
UBB.threads v6.5.3-v6.5.5
• MySQL versions 3.x and 4.x and will not run on MySQL version 5.x.
• PHP 4.1 and higher (4.3 and higher recommended)
UBB.threads v6.4-v6.5.2
• MySQL versions 3.x and 4.x and will not run on MySQL version 5.x.
• 4.0.1 and higher (4.1 and up recommended)
These can be now referenced at
UBBWiki Server Requirements and Suggestions - UBB.threads Historic Recommended System Versions
The recommended upgrade path to the latest build would be:
Upgrade UBB.Threads 6.4.x to UBB.Threads 6.5.5
Import UBB.Threads 6.5.5 into UBB.threads 7.2
Upgrade UBB.threads 7.2 to the latest build (7.7.x)
So you'd be looking at your project consisting of:
php 4.4.9 - ubbthreads-6-5-5.zip
php 5.2 to 5.6 - ubbthreads-7.0.0 to ubbthreads-7.6.2
php 5.4 to 7.4 - ubbthreads-7.7.0 to ubbthreads-7.7.4
Older builds of XAMPP will get you there
http://code.iamkate.com/articles/xampp-version-history-apache-mysql-php/PHP4 - 1.6.8
PHP5.4 - 1.8.2 for PHP5.4.27 or 1.8.3 for PHP5.5.11
I'm not sure at what build Xen requires for their importer, not my script.