When this framework was started, php4 was the only option. So over time, it evolved. It was important to us, and not very difficult, to keep compatibility with php4, so that applications written using old versions of the framework would continue to work. For example, zoop uses the autoinclude feature of php5 when available, but doesn't use it in php4. With the next version of Zoop, 2.0, php4 support will be dropped.

All that aside, I think it's naive to claim that something that works in php4 is bad by definition. PHP5 has some features that are very nice, but they aren't required to write programs, or to write good programs. They make it easier, but php4 had everything required to write perfectly acceptable code.