如何在php中将父类公共变量分配给子类公共变量

Question

I have created two class, Assigning public class variable to child class variable but not working properly.But the same variable assign working inside function correctly.I didn't get proper reason why like that.Please check below example.

      <?php
          Class pratice{
            public $a=4;
           }
         Class child extends pratice{
            //public $b =$this->a;//getting error with this assigment
            public function getValue(){
            $this->b = $this->a;//working fine with this
            echo $this->b;
           }
         }
        $obj = new child();
        $obj->getValue();
       ?>

Thanks in advance.

Answer 1

Well $this refers to an instance of the class, so until you create an instance of the class, $this isn't available.

You would have to do it in the constructor , when it can reference itself in a non-static context.

class Child extends Practice
{

    public $b;

    public function __construct()
    {
        $this->b = $this->a;
        echo $this->b;
    }
}