Abstract classes in php instantiate their subclasses in their methods to complete the call to their own methods

I am a newbie, if I encounter this, I can’t understand the beauty of it, record it;
The general logic is as follows

<code><span>abstract</span><span><span>class</span><span>A</span>
{</span><span>private</span><span>static</span><span>$c</span>;
    <span>final</span><span><span>function</span><span>todo</span><span>()</span>
    {</span><span>//do something</span>
    }
    <span>public</span><span><span>function</span><span>init</span><span>()</span>
    {</span><span>$c</span> = <span>new</span> B;
    }
    <span>public</span><span><span>function</span><span>get</span><span>()</span>
    {</span><span>$c</span>->todo();
    }
}
<span><span>class</span><span>B</span><span>extends</span><span>A</span>
{</span>}
....
A::get();</code>

Several questions arise here:
1. Can I create a new subclass in a parent class?
2. What are the considerations for doing this? Or what kind of problems is this design pattern avoiding?

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The above has introduced the abstract class in PHP that instantiates its subclass in its method to complete the call of its own method, including the content. I hope it will be helpful to friends who are interested in PHP tutorials.