On the "A Cloudy Place" blog there's a recent post comparing two of the more popular platform-as-a-service PHP offerings out there - PHPFog and PagodaBox.

Platform services like Heroku and AppEngine have been well-known for the last few years and many companies have been using them successfully in their business. They are more attractive to startups, due to low initial cost and smaller headaches involved in setup. 2011 was a great year for platform services, with the emergence of many platform services and a new trend for supporting multiple languages instead of supporting a single language. [...] Here, I will go through two of the PHP platform services I have experimented with: PHPFog and Pagoda Box.

He talks about each of the options and looks at several key elements of each including the deployment process, scalability, monitoring tools, offerings for caching and the database management utilities bundled with the application. Both services offer a "free instance" for you to try out their services, so give them both a try and see what you think.