I had to install a php ext and yum updated
all php 
from  php*5.3.6-1.el5.remi.x86_64
to    php*5.3.6-4.el5.remi.x86_64
and all mysql
from mysql*5.1.56-1.el5.remi.x86_64
to   mysql*5.5.14-1.el5.remi.x86_64

Unfortunately I have problems with mysql 5.5
and must downgrade back to mysql 5.1

What would be the best way to do it with yum  ?

Would the following work out ?
yum --enablerepo=remi remove php\* mysql*
yum --enablerepo=remi --exclude=mysql*5.5* install php* mysql*

Thank you in advance