The SitePoint PHP blog has a tutorial posted for the Heroku users out there showing you how to install custom PHP extensions on the service as a part of your deployment. Heroku is a platform-as-a-service hosting provider that allows for flexibility in the architecture of your systems and spin up/tear down to happen easily and on demand.
In this tutorial, we'll learn how to install custom extensions on Heroku. Specifically, we'll be installing Phalcon.
He walks you through creating an account on Heroku first and getting the Heroku toolbelt system installed for your operating system. He then starts in on the Phalcon (a C-based PHP framework) installation including all needed supporting packages/extensions. He uses the PHP buildpack and creates a shell script that is executed when the deployment happens. He includes the commands and configuration to handle the deployment and test the resulting installation.