There are two instances of PHP associated with Plesk. One is for Plesk admin's interface and the other for vhosts. You can't upgrade the admin PHP because it is very tightly integrated with Plesk, but you can easily upgrade the vhost PHP with the usual OS upgrade utilities like yum, apt-get, up2date, etc.
If your problem is maybe also that you don't have yum installed on your centos 5 - then visit openvz. This is no spam message - this was really the issue for me and I want to help anyone who'll have this problem.
working with a service provider's management interface, such as cPanel.