There is a list at the official PHP website: http://www.php.net/manual/en/refs.webservice.php You can see that PHP can work with OAuth, SCA, SOAP, Yar and XML-RPC. But there is another way which is so simple to create a webservice. It is JSON.
I will show how to create a simple web service with JSON. Our service will get the parameters by using $_GET , therefore we will put the parameters to url. For instance http://mywebserviceurl.com?s=myparameter , so the service will answer depending on myparameter.