Most CMS and Frameworks applications now require PHP 5.2 as the minimum requirement. Some have higher requirement to be able to function properly.
I have been in this situation when I installed Kohana Framework on my server. The latest Kohana requires PHP 5.3 to enable all its functionality. My VPS have only PHP 5.2.
As the my server OS is Centos 5, PHP 5.2 was installed by default. I needed to install PHP 5.3 to be able to use Kohana. Searching the net for the easy upgrade, I was able to find numerous upgrades with different levels of difficulties. The simpliest procedure I found is located at abdussamad.com. To summarize in three (3) steps:
1. yum update
2. yum remove php php-*
3. yum install php53 php53-cli php53-devel php53-gd php53-mbstring php53-mysql php53-pdo php53-xml php53-xmlrpc php-pear
The commands must be executed as a root user. After which, restart your server using the command
service httpd restart