Atomicorp

I am trying to upgrade an installation of php 5.1.6 to 5.2.3 on Centos 4.2. When I do
yum upgrade php
all seems to start out well, but then the process aborts with this message:
"Package php-imap-5.2.3-3.el4.art.i386.rpm is not signed".
I tried setting gpgcheck=0 in yum but this didn't help.
Does anyone have a solution to this problem?

Discovered that gpgcheck=1 was also in atomic.repo I changed that to 0, and everything ran until it said "done" but nothing was installed?

Deleted old version of php first, and then it worked!