The problem is that php 5.1 is already installed and running so
yum update php
I looked around the net for a solution but they say to do the above =/
You'll want to
php53 is available in CentOS as of CentOS 5.6 and above. It is not available in CentOS 5.5 and previous, unless you go through a third-party repository. That is why many answers on Serverfault tell you to use third party repositories-- that's not necessary anymore.
Note that php53 has many dependencies, so you should consider updating CentOS to the latest CentOS 5.x branch as part of this upgrade (which is CentOS 5.6 as of this writing). This is a major upgrade of PHP, and RedHat forced it through because they were way behind the times as of RHEL5.5.
Note how the package is named
You need any additional repo to upgrade php as php 5.3 is not yet included in the CentOS Base Repo. You can use the atomic repo which works for me flawlessly,
This will install the atomic repo, upgrade php and then remove the atomic-repo (to make sure that any future system update won't install any additional packages from atomic).