Red Hat Enterprise Linux is an excellent stable distro for mission critical enterprise environments. However the PHP and MySQL versions included are a tad behind mainstream. There is a great community repository from Rackspace that offers recent PHP, MySQL, and Python versions for Red Hat.
The project page is:
For Red Hat 5 and Red Hat 6, PHP 5.4.10 is available as well as MySQL 5.5.29.
I do recommend testing out any changes in development before pushing out to production, but these repositories are known to be very solid. Note that there is no support. Read over the FAQs for more information.