Communication between PHP web app server layers on Amazon Web Services?

There's a big chance the interfaces you have to develop for your application layer will be different for several apps. One way to have a generic approach for this is building more abstraction.

If you are using PHP as the backed then you are most likely going to have your webservers and "application" servers on the same machine (Apache + php) so the diagram from amazon doesn't really apply in this case. One thing that you may want to look into is putting a machine in front of your webservers (nginx or the like) to reverse proxy the requests back to your webservers, or maybe look into ELB from amazon.

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