PHP中的虚拟ICollection <classname>

I have a question herewith for the .net and PHP.

I am working with a application with Entity Framework in .Net and created few API (Webservices) which needs to be consumed by PHP Application.

Now I am trying to create the same replica of classes in PHP which I have created in .Net, to achieve the functionality I am looking for.

Also the objects are created on PHP page. but I am stuck at one point.

where in .Net I have created class as below

 public class User
 {
   Public User();
   public virtual ICollection<UserState> UserState { get; set; }
   public UserType UserType { get; set; }
 }

Now in above code, UserType is another class which I have created in PHP and created a Object for the same.

But for UserState I don't know how to do that code in PHP.

Can you please guide me on that?