davelister
The general approach would be
rpm -e php
and same for any other php5 dependencies you installed when you did the upgrade to php5 ie php.i386 php-pear-Net-Socket php-pear-Auth-SASL php-pear-DB php-pear-HTTP php-pear-Mail php-pear-XML-Parser
If you remove the packages one by one, you will need to uninstall the rpms in the correct order to avoid dependency issues.
To uninstall dependencies more easily, let rpm do it in the one command
rpm -e php php.i386 php-pear-Net-Socket php-pear-Auth-SASL php-pear-DB php-pear-HTTP php-pear-Mail php-pear-XML-Parser
The above command is all on one line
To force the package removal, ignoring dependencies
rpm -e --nodeps php
You should always use the above command very carefully and only if you know the full ramifications of doing so. The nodeps switch should not normally be used.
Then to get back to the current version
yum update php
signal-event post-upgrade
signal-event reboot