There are updated packages for php version 5.1.6-1.2 in the c4-testing repository available at dev.centos.org for i386 and x86_64 arches.
Also included is an update to php-pear. This update should also resolve the direct install issue experienced by some where php-pear conflicted with the package in the base repository.
Please test these and report any issues you might have, as these will hopefully be the upgrade path for php5 in centosplus. Any feedback is welcomed, either via replying to this message, or by adding to the php5 tracking bug at http://bugs.centos.org/view.php?id=1580
If you don't have it already... http://dev.centos.org/centos/4/CentOS-Testing.repo