At Google IO this year, Google announced support for a new runtime running on App Engine, PHP. Currently, they have full support for languages such as Python, Java, and Go.
PHP on App Engine was the number one request voted by developers and Google has finally delivered. PHP empowers a great percentage of the web sites in the world, it is very likely this will be taken out of Experimental mode soon into full Production mode.
Google has taken the open source PHP 5.4 platform and augmented it to run on App Engine. They created a safe "sandboxed" environment for your apps.
A match made in heaven
PHP is an inherent scalable language. It's thread-safe and stateless nature, makes an ideal candidate to run on scalable, elastic infrastructure. PHP allows you to code rapidly and App Engine allows you to deploy rapidly, the combination makes an incomparable development platform.
Since Google is implementing PHP 5.4, their PHP runtime has the built-in extensions for APC, OpenSSL, OAuth, among others. APC is an essential component to any PHP installation, so it's ideal that they jumped the gun and started with PHP 5.4 and 5.3.*.
What this means for PHP developers?
For a number of years now (since 2008), Java developers have enjoyed and exploited the power of Google's cloud to create their own applications. This is the same infrastructure that Google uses to contain their own applications and tools, so it's proven to be super scalable and reliable. App Engine falls into the category of Platform-As-A-Service (PaaS) that, coupled with great tools, endows your development environment with great agility. Now, PHP developers can take advantage of this.