Another package that requires recompiling after the php 5.1.X upgrade is eAccelerator. Any version prior 0.9.5 will not work for php 5.1. Without any src.rpm or binary rpm available anywhere, I have repackaged it based on the source rpm from DAG repo, you can download these packages compiled with php 5.1.X in CentOS4 or any compatible distributions:

eAccelerator 0.9.5 SRPMS
eAccelerator 0.9.5 x86_64 RPM

If you receive dependency errors when you install the binary package, it means your environment or libraries may be different than mine. Just grab the source RPM and run “rpmbuild –rebuild php-eaccelerator-5.1.6_0.9.5-1.lh.src.rpm” to rebuild it against your libraries.

Since the upgrade from PHP 5.0.4 to PHP 5.1.6, the wp-cache plugin in wordpress failed on any pages that were not cached before. The page shows blank screen on the first run - it works after it cached once on the page. I did some quick searches and found this solution for wp-cache version 2.0.19 with php 5.1.X.

Out of my curiosity, I searched for this behavior changes from PHP 5.0 to 5.1 and found this bug report.