PHP Using Factory pattern for SDKs

I'm a bit lost here because I want to do something that is very easy in Java but seems a bit complicated in PHP.

We are building an SDK for our product and in Java, we have this one class that must not (!) be instantiated by the user (i.e. the coder), since there are several constraints regarding it's integrity. So we've built that as a nested class "X" inside of the "XFactory" and you will get an instance of X by calling XFactory.buildMeMyX(); - Easy...

Now PHP does not support nested classes at all, and I wonder how to apply the same here. In Java, X's constructor is hidden (private), so only XFactory can call it.

In PHP, it looks like I will have to make __construct() public and move the nested class X out of XFactory. Hence, the user will be able to create an instance without the Factory.

Now - I COULD move the factory functionality to X itself and move all the stuff there, but this would kind of break the design of the SDK. Is there a useful way to do such things in PHP after all?