Please note that these documents apply to OpenShift Online v2 only (accounts created before August 1st, 2016).
If you are using one of the OpenShift 3 products, please refer to documents available at

Zend Server Application Hosting

Zend Server is an enterprise application platform for PHP applications. OpenShift Online currently supports Zend Server 6.1.

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The Zend Server Cartridge

The zend-server cartridge provides support for deploying PHP applications on OpenShift, using the Zend Server. A basic PHP cartridge is also available.