如何避免无休止的构造循环？

I have two classes that depend on each other.

class ClassA extends SomeClass
{
    public function __construct(ClassB $classB) {
        $this->classB = $classB;
    }
}

class ClassB extends SomeOtherClass
{
    public function __construct(ClassA $classA) {
        $this->classA = $classA;
    }
}

How can I avoid an endless loop as they're constructed?

There won't be any loop. Neither of them instantiate a new object in their constructors.