I managed to install mysql5 and php5 on sme 7.2 without problems
rpm -e --nodeps php-domxml
db yum_repositories setprop centosplus Exclude kernel,kernel-smp,mkinitrd,mdadm,initscripts,perl*,perl-DBI,perl-DBD-MySQL,httpd,mod_ssl
expand-template /etc/yum.conf
yum update php mysql \
--disablerepo=* \
--enablerepo=base \
--enablerepo=updates \
--enablerepo=centosplus \
--enablerepo=smeos \
--enablerepo=smeupdates \
--enablerepo=smeaddons
signal-event post-upgrade
signal-event reboot
mysql_upgrade
signal-event post-upgrade
signal-event reboot
server-manager, phpmyadmin, webmail all running fine
HTH
Ciao
Stefano