Abstract classes in PHP instantiate their subclasses in their methods to complete the invocation of their own methods

Beginner, encounter this, can not realize the beauty of the, record;
The broad logic is as follows

abstractclassA{privatestatic$c;    finalfunctiontodo()    {//do something    }    publicfunctioninit()    {$cnew B;    }    publicfunctionget()    {$c->todo();    }}classBextendsA{}....A::get();

Here are some questions that arise:
1. Can a subclass be new in the parent class?
2. What are the considerations for doing so? Or does this design pattern prevent problems from appearing?

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The above introduces the abstract class in PHP in its methods to instantiate its subclasses to complete the invocation of their own methods, including aspects of the content, I hope that the PHP tutorial interested in a friend helpful.

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