Google Cloud Engine provides different version of CentOS. One is CentOS 6x, the other is CentOS 7. Both version 6x and 7 provide the PHP 54. If you need upgrade the PHP from PHP54 to PHP56, the following steps can work for me: (My environment is CentOS 7)
Step 1: Install EPEL (Skip this if EPEL installed!)
wget http://rpms.famillecollet.com/enterprise/remi-release-7.rpm
rpm -Uvh remi-release-7*.rpm epel-release-7*.rpm