Inheritance in PHP

<?php

 /**
 * inheritance in php
 */
 class Grandfather
 {
     // global varible 
     var $global_variable;

     // constructor of grandfather
     function __construct()
     {
         $this->global_variable = 56;
         echo "I am grandfather 
";
     }

     function default_function()
     {
         echo "this is default function in grandfather 
";
     }

     private function private_function()
     {
         echo "this is private function in grandfather 
";
     }

     protected function protected_function()
     {
         echo "this is protected function in grandfather 
";
     }

     public function public_function()
     {
         echo "this is public function in grandfather 
";
     }
 }

 /**
 * this is sub class which is extending Grandfather class
 * this will inherit all property except private property
 */
 class Father extends Grandfather
 {
     // global variable
     var $father_var;
     function __construct()
     {
         // equivalent to => parent::__condtruct();
         Grandfather::__construct();
         $this->father_var = 256;
         echo "I am father 
";
     }

     public function display_all()
     {
         $this->default_function();
         $this->protected_function();
         $this->public_function();
         echo "I am father's display_all 
";
         parent::public_function();
     }
 }

 /**
 * this is child class which is inheriting from father
 * this is indirectly also inheriting from grandfather
 * just like you because father's property includes 
 * grandfather's property
 */
 class Child extends Father
 {
     // constructor in Child
     function __construct()
     {
         Grandfather::__construct();
         echo "I am child 
";
     }

     // child is modifying father's funciton
     // it is called overriding
     function display_all()
     {
         echo "function from father
";
         // including father's display function
         parent::display_all();
         echo "new added in child
";
     }
 }
 $obj = new Father();
 $obj->display_all();
 
 echo "


Child object call

";
 $obj2 = new Child();
 $obj2->display_all();

?>

I am grandfather 
I am father 
this is default function in grandfather 
this is protected function in grandfather 
this is public function in grandfather 
I am father's display_all 
this is public function in grandfather 
Child object call

I am grandfather 
I am child 
function from father
this is default function in grandfather 
this is protected function in grandfather 
this is public function in grandfather 
I am father's display_all 
this is public function in grandfather 
new added in child