References in Objects and Arrays

PHP References - Part 3

Foreword: In this part of the series, I talk about References in Objects and Arrays.

By: Chrysanthus Date Published: 2 Dec 2018

Introduction

This is part 3 of my series, PHP Reference. In this part of the series, I talk about References in Objects and Arrays. You should have read the previous parts of the series before coming here, as this is the continuation.

Reference as a Property
A reference can be the property of a class. Try the following code:

<?php

 $vr = 'I love you.';

 class Cla
 {
 public $prop;

 function __construct()
 {
 global $vr;
 $this->prop = &$vr;
 }
 }

 $obj = new Cla();
 $vr = 'I hate you.';
 echo $obj->prop;

?>

In the creation of the object, the construction assigns a reference to the property. Outside and below the class definition, the content of the reference is changed. The output is:

 I hate you.

Copying an Object
When copying an object to a variable, the references to the object properties are not copied - they cannot be copied and should not be copied. The official syntax to copy an object in PHP is:

 $copy_of_object = clone $object;

Try the following code, which shows that the reference to an object property has not been copied:

<?php

 $vr = 'I love you.';

 class Cla
 {
 public $prop;

 function __construct()
 {
 global $vr;
 $this->prop = &$vr;
 }
 }

 $obj = new Cla();
 echo $obj->prop, ' ';
 $vr = 'I hate you.';
 echo $obj->prop, ' ';
 $obj2 = clone $obj;
 $vr = 'Why?';
 echo $obj2->prop;

?>

The output is:

 I love you.
 I hate you.
 Why?

Reference as Value of Element in Array
A reference can be the value of an element in an array. Try the following code:

<?php

 $vr = "yellow";

 $arr = array('Apple' => "purple", 'Banana' => &$vr, 'Pear' => "green");

 $vr = "white";
 echo $arr['Banana'];

?>

A reference has been assigned as value to the key, 'Banana'. The output is:

 white

Copying an Array
When copying an array to a variable, references to keys are not copied - they cannot be copied and should not be copied. The syntax to copy an array in PHP is just:

 $copy_of_array = $array;

Try the following code, which shows that the reference to a key has not been copied:

<?php

 $vr = "yellow";

 $arr = array('Apple' => "purple", 'Banana' => &$vr, 'Pear' => "green");

 echo $arr['Banana'], ' ';
 $vr = "white";
 echo $arr['Banana'], ' ';
 $arr2 = $arr;
 $vr = "black";
 echo $arr['Banana'], ' ';

?>

The output is:

 yellow
 white
 black

Object Reference
An object variable can be referenced. When this is done, the references inside the object remain. Try the following code:

<?php

 $vr = 'I love you.';

 class Cla
 {
 public $prop;

 function __construct()
 {
 global $vr;
 $this->prop = &$vr;
 }
 }

 $obj = new Cla();
 $obj1 = &$obj;
 echo $obj1->prop, ' ';
 $vr = 'I hate you.';
 echo $obj1->prop, ' ';
 $obj2 = clone $obj1;
 $vr = 'Why?';
 echo $obj2->prop;

?>

The output is:

 I love you.
 I hate you.
 Why?

Array Reference
An array variable can be referenced. When this is done, the references inside the array remain. Try the following code:

<?php

 $vr = "yellow";

 $arr = array('Apple' => "purple", 'Banana' => &$vr, 'Pear' => "green");

 $arr1 = $arr;
 echo $arr1['Banana'], ' ';
 $vr = "white";
 echo $arr1['Banana'], ' ';
 $arr2 = $arr1;
 $vr = "black";
 echo $arr['Banana'], ' ';

?>

The output is:

 yellow
 white
 black

That is it for this part of the series.

Chrys

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