How to obtain the data returned by a method in the remote class

How do I get the data returned by a method in the remote class? If it is local, the return method can be obtained directly under the new class, but if it is two different websites, classlist {publicfunctionindex () {return123 ;}} in the above example, in the local newlist, how can we get the data returned by a method in the remote class?
If it is local, the return method can be obtained directly under the new class, but what if it is two different websites?

Class list {
Public function index (){
Return 123;
}
}

In the above example, you can create a new list locally and then call index.

------ Solution --------------------
Just write a php interface remotely... Data can be encapsulated in json or xml.