php: change php code for service and type classes

Service classes now have to call BaseType::toRequestXml as the
visibility of BaseType::toXml has been changed from `public` to
`private`.

Update the ctor code for type classes to set the
`$requestXmlRootElementNames` member.
php: change code for service objects calling callOperation

The method parameters of callOperation has changed. Service object must
now pass an instance of the BaseType class as the second parameter.