Those are really two entirely different things: PHP based web services, and Android development that uses web services. If you study them like that; separately, you'll get much better results than if you try to find matterial where both are linked together.
PHP doesn't really care what type of client-side device is requesting the web services it provides. Whether it's a normal browser or an Android app, both are handled the same by PHP. Likewise, the Android app doesn't care what language is providing the web service, as long as the response contains the expected content.
On the PHP side, there are any number of ways to set up a web service. Restler is one of them, but there are a LOT of different ways to handle this type of thing.
I don't know much about Android, but I'd assume the Java APIs for it include classes to handle HTTP communication and, at the very least, ways to read XML data. Perhaps even REST specific APIs and JSON parsing.
you have to learn how to create web services in php (JSON and XML) then learn how to consume the web service in your android App, as someone said above you can create your app with html, css and js but if you want to go the extra mile learn java