Question

<?php

$instance = new SimpleClass();

$assigned   =  $instance;
$reference  =& $instance;

$instance->var = '$assigned will have this value';

$instance = null; // $instance and $reference become null

var_dump($instance);
var_dump($reference);
var_dump($assigned);
?> 

what is the difference between below given 2 lines?

$assigned   =  $instance;
$reference  =& $instance; 

as in OOP object is by default assign by reference . so $assigned will also have
& $instance.

output of above code is

NULL
NULL
object(SimpleClass)#1 (1) {
   ["var"]=>
     string(30) "$assigned will have this value"
}

Answer 1

as in OOP object is by default assign by reference

This is not totally true.

What the $instance holds is the value of the object id, when you assign an object to another variable, you only passed the object id.

So when you do $assigned = $instance;, you are passing the object id which $instance holds to the variable $assigned, and $instance = null is only set the variable $instance to null, nothing will affect to $assigned.

But when you do $reference =& $instance, you are creating a reference to the variable $instance, so if you set $instance to null, $reference will also be null.