DigitalOcean has an API for creating and destroying droplets and replicating droplet control panel functionality. The API is RESTful, uses oAuth, supports IPv6 and comes with an extensive documentation.
Heroku is one of the oldest PaaS providers. The fact that it's been around for such a long time means that it had a lot of time to mature over the years. There's also a massive number of articles, guides and tutorials on Heroku out there for beginners and advanced users.
Google App Engine give you a truly managed environment with very high security and simple management tools. No stack to manage or worry about - you are truly running in the cloud, not managing a remote real or virtual server.
You do not have direct control of the file system or the web server - this is great for security, but transporting some existing PHP packages that rely on redirection hacks can require a bit of learning.
Aldryn Cloud takes django CMS a step further by facilitating modern, independent collaboration practices. It’s designed with real-time collaboration in mind and to help increase work productivity. Aldryn allows you to work with two classes of user, Collaborators and Clients (CMS-only users).