Please read this CentOS wiki on yum priorities. That should help you set up conflicting 3rd party repositories. [by toracat]
Yes, and CentOS wiki Repositories will help you understand the "3rd party repositories". In your case... [by toracat]
It would appear that the version I have of php-common-5.2.10-1.el5.centos is newer than the one being asked for as a dependency of php-xml when I try to install php-xml using yum install php-xml.