I would like to create a web service in PHP which can be consumed by different consumers (Web page, Android device, iOS device).
I come from a Microsoft background s
PHP does have native support for a SOAP server ( The SoapServer class manual shows it) and I've found it pretty simple to use.
Creating a REST style API is pretty easy if you use a framework. I don't want to get into a debate about which framework is better but CakePHP also supports output as XML and I'm pretty sure others will as well.
If you're coming from a Microsoft background just be careful about thinking about "datasets". They are a very specific Microsoft thing and have been a curse of mine in the past. It's probably not going to be an issue for you, but you may want to just see the differences between Microsoft and open implementations.
And of course PHP has a native json_encode() function.