It's not as simple as "yum install" as you can see below and it doesn't stop there.
yum install php53
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* rpmforge: mirror.us.leaseweb.net
* extras: centos.mirror.rafal.ca
* updates: centos.mirror.nexicom.net
* base: centos.mirror.nexicom.net
* addons: centos.mirror.nexicom.net
Setting up Install Process........
PHP5 Centos 6 displays part of code from PHP file for some scripts
edit /etc/php.ini change short_open_tag = Off to:
short_open_tag = On
Then restart Apache and it should be resolved (assuming the scripts failing used )