It’s a popular question. Why indeed should one pick PHP over one of the alternatives?

Advantages:

• Virtually infinite scalability
  If your site gets a peek of requests, in theory AppEngine will handle the load. In a regular PHP setup you would need to take care of adding more servers and distribute the requests between them.
• Automated backups
  Your application and database data are backed by the AppEngine itself. You can make it take a backup on demand or even schedule it.
• Distributed sessions
  AppEngine emulates a memcached server, so you can store and retrieve data that will be available to all scripts of your application regardless if they are running on the same machine or not. This is useful for instance to store and retrieve session data. This is better than the default session storage that PHP uses which is based in local files.
• Task queues
  AppEngine provides access to a queue that you can use to dynamically defer the execution of a script to a background process. A class is provided to make it easy to queue the execution of background PHP scripts.Your application can also schedule tasks to be executed regularly like you would do configuring a crontab file.