IBM's cloud offerings are all built on SoftLayer, which provides cloud infrastructure as a service. You can customize your instance to suit your PHP application requirements, and then easily install Zend Server.
If building your own cloud environment from the ground up isn't your core competency, you can leverage IBM PureApplication Patterns to do some of the work for you. Patterns have been available as a private cloud solution since 2012, and now they are available on SoftLayer as well.
BlueMix is IBM's Platform as a Service (PaaS) offering based on Cloud Foundry, an industry standard for PaaS. With BlueMix, you select Zend Server for your PHP runtime, then select the services your application needs, including database, single sign on, and message queuing.
Whether you want to run Zend Server on a bare metal server or as a runtime service in BlueMix, you can buy Zend Server for all of IBM's cloud offerings on the IBM Marketplace, a one-stop shop for all the applications, runtimes, patterns, and services available on IBM SoftLayer.