To update to php 5.4 and mysql 5.5 in centos 6:

1) install the epel, remi and rpm-forge repos

rpm -Uvh http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm
rpm -Uvh http://rpms.famillecollet.com/enterprise/remi-release-6.rpm
rpm -Uvh http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el6.rf.x86_64.rpm

2) edit the /etc/yum.repos.d files to enable the repos

vi /etc/yum.repos.d/epel.repo

change the enabled=0 to enabled=1

vi /etc/yum.repos.d/remi.repo

change the enabled=0 to enabled=1

3) install php and mysql

yum install php mysql

4) you now have php 5.4 and mysql 5.5
5) you can do a yum update to update anything else that needs updating (yum update)
6) restart apache (service httpd graceful)