Your PHP server should provide an API for the network. It does not have to be a specific API or technology (SOAP, JSON, REST, XML-RPC, etc.). Almost everyone will work as long as you agree to one technique. Having said that, REST api with JSON-encoded data is especially suitable for Android mobile applications.
Here is an article about creating a REST API in PHP for your server: http://www.gen-x-design.com/archives/create-a-rest-api-with-php/
In one of these years, Google IO talks about consuming the REST API on Android: http://code.google.com/events/io/2010/sessions/developing-RESTful-android-apps.html
These two links should at least get started.
API - , , PHP, , , .
, , Android .