To install, first you must add the Webtatic EL yum repository information corresponding to your CentOS/RHEL version to yum:
CentOS/RHEL 6.x:

rpm -Uvh https://dl.fedoraproject.org/pub/epel/epel-release-latest-6.noarch.rpm
rpm -Uvh https://mirror.webtatic.com/yum/el6/latest.rpm

Now you can install PHP 5.5’s mod_php SAPI (along with an opcode cache) by doing:
yum install php55w php55w-opcache


You can alternatively install PHP 5.5’s php-fpm SAPI (along with an opcode cache by doing:
yum install php55w-fpm php55w-opcache

If you know what you are doing, you can upgrade PHP by:
yum install yum-plugin-replace
 
yum replace php-common --replace-with=php55w-common

Opcode Caches

The PHP distribution now comes with an opcode cache. This is the Zend Optimizer+ opcode cache, now known as the Zend OPcache extension. This extension is optional, so does not preclude you from using an alternate one. APC has been less maintained in recent years, so not as suitable (however it did also have a very convenient in-process user-cache which will be missed).

Due to it being included in the PHP source distribution, it will be well maintained and more suitable for use while other Opcode cache’s are being updated over the coming months.

 

yum install php55w-opcache