Question

What is the difference between two codes?

Extends;

<?php 
require_once 'example.class.php';
Class First extends Example
{}
?>

Normal calling;

<?php
require_once 'example.class.php';
Class First
{
    public $example;
    function __construct()
    {
        $this->example = new Example();
    }
}
?>

I know some difference for example using protected pharase.
But this is not enough in my opinion.

Answer 1

The first one, the object First will have the same properties/functions of Example class. Like:

class Example
{
    public function a()
    {
    }
}

class First extends Example
{
    public function b()
    {
    }
}

if you instance two objects $ex1, $ex2:

$ex1 = new Example();
$ex1->a(); // this is valid
$ex1->b(); // this is invalid because Example doesn't have "b" function

$ex2 = new First();
$ex2->a(); // this is valid
$ex2->b(); // this is valid too, because First inherits Example members + its own

on the second code, you’re creating an instance of example so you must access that variable to be able to call Example method.

one better example:

class Person
{
    public $name;

    public function say($message) 
    { 
        echo $this->name . " says " . $message;
    }
}

class Teacher extends Person
{       
    public function say($message) 
    { 
        // note that Teacher has a name even this is not declared here.
        echo $this->name . " says " . $message; 
    }

    public function teach($what) 
    { 
        // note that Teacher has a name even this is not declared here.
        echo $this->name . " is teaching " . $what; 
    }
}

See the output:

$john = new Person();
$john->name = "John Doe";
$john->say("hello world!");
/* 
$john->teach("Portuguese"); // invalid, person doesn't teach anything.
*/ 


$chuck = new Teacher();
$chuck->name = "Chuck Norris";
$chuck->say("hello universe!");
$chuck->teach("Fighting"); // valid because Teacher has method "teach"