This article demonstrates how to upgrade from php-5.3 to php-5.5 on CentOS or Redhat.
rpm -Uvh https://mirror.webtatic.com/yum/el6/latest.rpm
Now run the yum command to see a list of php-5.5 options:
yum list php55w*
Run the upgrade using the following commands:
yum install yum-plugin-replace yum replace php-common --replace-with=php55w-common
You will receive a warning message. Saying YES has never caused me an issue. You will get the opportunity to see what will be removed and what will be installed after choosing YES.
At this point you should be able to run the following command to verify the upgrade. Note the version number:
php -v