Notice: Undefined property PHP in inherited class

I am studying PHP, and a problem arises (Notice: Undefined property) when I do the HERENCIAS exercise of the class.

More specifically, he says:

  

"Notice: Undefined property: double-inherited :: $ muninterno in   C: \ xampp \ htdocs \ project_php_seas \ 418-herencias.php on line 25 ".

If someone can help me, I would be grateful, since my teachers are cows.

I put the code in PHP and then the result:

************** PHP code: *************

    <?php
    class doble {

      var $mult=2;
      public $numinterno;



      public function multxnum() {
        return $this->mult*$this->numinterno;
      }

      public function divxnum () {

        return $this->numinterno/$this->mult;
      }

    }


    class dobleheredada extends doble {


      public function triple() {
        return ($this->multxnum()*$this->muninterno);
      }
    }

    $miobjeto= new doble();
    $miobjeto->numinterno=3;
    $eldoble=$miobjeto->multxnum();

    echo "El valor doble del parámetro es: $eldoble";
    echo "<br>";

    $lamitad=$miobjeto->divxnum();
    echo "El valor mitad del parámetro es: $lamitad";

    $objeto3=new dobleheredada();
    $objeto3->numinterno=3;
    $eltriple=$objeto3->triple();

    echo "<br>";
    echo "El doble del cuadrado del parámetro es: ".$objeto3->triple();
    echo "El doble del cuadrado del parámetro es: $eltriple";

    ?>

************* Script Result: **********

  

The double value of the parameter is: 6

     

The half value of the parameter is: 1.5

     

Notice: Undefined property: double-inherited :: $ muninterno in   C: \ xampp \ htdocs \ project_php_seas \ 418-herencias.php on line 25

     

Notice: Undefined property: double-inherited :: $ muninterno in   C: \ xampp \ htdocs \ project_php_seas \ 418-herencias.php on line 25 El   double the square of the parameter is: 0 Double the square of the   parameter is: 0 "