References and Variables

PHP Tutorials - Herong's Tutorial Examples

∟References and Variables

This section describes what is a reference, creating references with & operator, assigning references to variables, removing reference links with unset($var) function.

What is a reference? A reference is an alias of a variable. PHP supports references in a very similar way as the Perl language. Here are some basic rules about using references in PHP:

1. To create a reference of a given variable, you need to use the reference operator: &. For example, &$title creates a reference of variable $title.

2. Reference can be assigned to another variable using the assignment operator: =. For example, $subject = &$title assigns the reference of variable $title to variable $subject.

3. The variable holding the reference is an alias of the original variable and behaves the same way as the original variable. For example, $subject = &$title; var_dump($subject); prints information of the data assigned to variable $title.

4. If the original variable is assigned to a new data, the reference variable is automatically assigned to that new data. For example, $subject = &$title; $title = "New String"; assigns "New String" to both $subject and $title.

5. If the reference variable is assigned to a new data, the original variable is automatically assigned to that new data. For example, $subject = &$title; $subject = "New Text"; assigns "New Text" to both $subject and $title.

6. Multiple reference variables can be created by assigning the reference to multiple variables. For example, $subject = &$title; $topic = &$title; assigns the reference of $title to both $subject and $topic.

7. Actually, original variable and its reference variables can all be viewed as references to the assigned data shared by all of them. For example, $subject = &$title; $topic = &$title; $topic = "New Message"; creates 3 variables referring to the same data "New Message".

8. To remove the reference link between a variable and its assigned data, you need to use the unset($var) function. For example; $title = "New String"; unset($title); removes the reference link between $title and "New String". $title is in the "unset" states now.

9. If multiple variables are referencing the same data, removing the reference link on one variable does not affect other variables. For example, $subject = &$title; $topic = &$title; $topic = "New Message"; unset($topic); removes the reference link on $topic, but $subject and $title are still referring to "New Message".

10. If multiple variables are referencing the same data, assigning a new reference to a new data to one variable does not affect other variables. For example, $subject = &$title; $topic = &$title; $topic = "New Message"; $topic = &$name; assigns the reference of $name to $topic, but $subject and $title are still referring to "New Message".

To show you some of reference rules mentioned above, I wrote the following PHP script, ReferenceTest.php:

<?php
#  ReferenceTest.php
#- Copyright 2003 (c) HerongYang.com. All Rights Reserved.
#
   $title;
   print "\n \$title is not assigned to anything:\n";
   print "    \$title: "; var_dump($title);

   $subject = &$title;
   print "\n \$subject is an alias of $title:\n";
   print "    \$subject: "; var_dump($subject);
   print "    \$title: "; var_dump($title);

   $subject = "Herong's PHP Book";
   print "\n \$subject is assigned with a string:\n";
   print "    \$title: "; var_dump($title);
   print "    \$subject: "; var_dump($subject);

   $title = "Herong's Programming Book";
   print "\n \$title is reassigned with a new string:\n";
   print "    \$title: "; var_dump($title);
   print "    \$subject: "; var_dump($subject);

   $topic = &$subject;
   print "\n \$topic is added as the third reference variable:\n";
   print "    \$title: "; var_dump($title);
   print "    \$subject: "; var_dump($subject);
   print "    \$topic: "; var_dump($topic);

   unset($subject);
   print "\n \$subject's reference link is removed:\n";
   print "    \$title: "; var_dump($title);
   print "    \$subject: "; var_dump($subject);
   print "    \$topic: "; var_dump($topic);


   $name = "Herong's Tutorial Book";
   $title = &$name;
   print "\n \$title is assigned to a new reference:\n";
   print "    \$title: "; var_dump($title);
   print "    \$subject: "; var_dump($subject);
   print "    \$topic: "; var_dump($topic);
   print "    \$name: "; var_dump($name);
?>

If you run this sample script in PHP 7 and higher, you should get output with some "Notice" messages. This is because now the default message level is set to E_NOTICE. You can change the setting or modify the code to make sure all variables are defined before using them.

herong> \php-7.3\php ReferenceTest.php

 $title is not assigned to anything:
    $title: PHP Notice:  Undefined variable: title in
       C:\herong\ReferenceTest.php on line 7

Notice: Undefined variable: title in
   C:\herong\ReferenceTest.php on line 7
NULL

 $subject is an alias of :
    $subject: NULL
    $title: NULL

 $subject is assigned with a string:
    $title: string(17) "Herong's PHP Book"
    $subject: string(17) "Herong's PHP Book"

 $title is reassigned with a new string:
    $title: string(25) "Herong's Programming Book"
    $subject: string(25) "Herong's Programming Book"

 $topic is added as the third reference variable:
    $title: string(25) "Herong's Programming Book"
    $subject: string(25) "Herong's Programming Book"
    $topic: string(25) "Herong's Programming Book"

 $subject's reference link is removed:
    $title: string(25) "Herong's Programming Book"
    $subject: PHP Notice:  Undefined variable: subject in
       C:\herong\ReferenceTest.php on line 33

Notice: Undefined variable: subject in
   C:\herong\ReferenceTest.php on line 33
NULL
    $topic: string(25) "Herong's Programming Book"

 $title is assigned to a new reference:
    $title: string(22) "Herong's Tutorial Book"
    $subject: PHP Notice:  Undefined variable: subject in
       C:\herong\ReferenceTest.php on line 41

Notice: Undefined variable: subject in
   C:\herong\ReferenceTest.php on line 41
NULL
    $topic: string(25) "Herong's Programming Book"
    $name: string(22) "Herong's Tutorial Book"

If you run this sample script in PHP 5 and lower, you should get output without any "Notice" message:

herong> \php-5.6\php ReferenceTest.php

 $title is not assigned to anything:
    $title: NULL

 $subject is an alias of :
    $subject: NULL
    $title: NULL

 $subject is assigned with a string:
    $title: string(17) "Herong's PHP Book"
    $subject: string(17) "Herong's PHP Book"

 $title is reassigned with a new string:
    $title: string(25) "Herong's Programming Book"
    $subject: string(25) "Herong's Programming Book"

 $topic is added as the third reference variable:
    $title: string(25) "Herong's Programming Book"
    $subject: string(25) "Herong's Programming Book"
    $topic: string(25) "Herong's Programming Book"

 $subject's reference link is removed:
    $title: string(25) "Herong's Programming Book"
    $subject: NULL
    $topic: string(25) "Herong's Programming Book"

 $title is assigned to a new reference:
    $title: string(22) "Herong's Tutorial Book"
    $subject: NULL
    $topic: string(25) "Herong's Programming Book"
    $name: string(22) "Herong's Tutorial Book"