For CentOS 6, PHP 5.3.3 is the latest version of PHP available through the official CentOS package repository. Keep in mind, even though PHP 5.3.3 was released July 22, 2010, the official CentOS 6 PHP package was updated November 24, 2013. Why? Critical bug fixes are backported. See this question for more information: "Why are outdated packages installed by yum on CentOS? (specifically PHP 5.1) How to fix?"
If you'd like to use a more recent version of PHP, Les RPM de Remi offers CentOS PHP packages via a repository that you can add to the yum package manager. To add it as a yum repository, follow the site's instructions.
Note: Questions of this variety are probably better suited for Server Fault.