I've played a lot today with yum and found that php and mysql can be updated to the 5 series from the centoplus repo without having to mess around with php-domxml as per the howto.
http://bugs.contribs.org/show_bug.cgi?id=1120It appears the horde dependency on php-domxml is removed. During an update of a test server php-domxml was erased and php-xml installed.
Webmail continued to function.
I believe I did an update to get it..
yum --enablerepo=base --enablerepo=updates --enablerepo=centosplus update
This last line may be your problem...
NOTE: This extension has symbol clashes with the php-dom package, do not use
both at the same time.
You did not state how you got the '5' series on there in the first place.