To do this, you'd issue a command like 'yum --enablerepo centosplus
install php php-pear'

Thanks!  I'm reading over all the details on the following URL:

http://mirror.centos.org/centos/4/centosplus/Readme.txt

This section was especially helpful:

 
-------------------------
php-5.x.x-x.centos4
-------------------------
This is php-5 for CentOS-4. It replaces the php-4.3.9 that is currently
in CentOS-4.  In order to upgrade a current php-4 install, you must
remove php-domxml first and then do:

yum --enablerepo=centosplus upgrade php*

or to do and install, use this command:

yum --enablerepo=centosplus --exclude=php-domxml install php*

(or install individually the files that you need)