Using references in PHP: The Basics

Written by: Phil Sturgeon

Hello, I am Phil and I have been developing websites since 1999. I started my career freelancing then ran a small web development team for over a year. Now I work as a developer for HargreavesLansdown and use my spare time to develop, write articles about programming and create slightly intoxicated screencasts. For more of my antics follow me on twitter @PhilSturgeon.

This is the first post of a three part series that will explain what the hell reference variables (pointers) are and you can use them in your PHP projects.

To quote the PHP Manual “References in PHP are a means to access the same variable content by different names.” which I believe is a perfect explanation.  So, If you pass by reference instead of making a copy you are modifying the original variable. This is done by placing a single “&” (ampersand) in front of the variable you are assigning.

The concept sounds a little strange at first, but lets take a look at the example.

<?php
// Create a variable.
$original_variable = 1;

// Create a reference to $original_variable
$ref_variable =& $original_variable;

// Increment $ref_variable by 10
$ref_variable += 10;

// Output $original_variable
echo $original_variable; // Outputs "11"

// Decrement $original_variable by 5
$original_variable -= 5;

// Output $ref_variable
echo $ref_variable; // Outputs "6"

?>

I know what you are thinking… “what the hell?!” right? Ok, lets take that one a little slower.

// Create a variable.
$original_variable = 1;

Just making avariable.

// Create a reference to $original_variable
$ref_variable =& $original_variable;

$ref_variable is now a “clone” or reference of $original_variable. Any modifications made to either will reflect in both variables as they are essentially the same variable with different names. The one exception to this rule is deleting. If you delete $ref_variable, $original_variable will still exist.

// Increment $ref_variable by 10
$ref_variable += 10;

// Output $original_variable
echo $original_variable; // Outputs "11"

This line will add 10 to $ref_variable which of course means the value of $original_variable will go up by 10.

// Increment $original_variable by 10
$original_variable -= 5;

// Output $ref_variable
echo $ref_variable; // Outputs "6"

Finally we have a similar block of code. The purpose of this was to demonstrate that you can modify either variable to change them both. Changes to the $original_variable will reflect in $ref_variable as soon as they are made.

One thing to remember is that you can only create references to variables. That means any static strings, ints, floats, etc will throw an error.

<?php
// Try and create a new reference:
$ref_variable =& 10;
?>

That code will throw a syntax error like “Parse error: syntax error, unexpected T_LNUMBER, expecting T_NEW or T_STRING or T_VARIABLE or ‘$’”, to correct it the value should first be assigned to a variable and then a reference made to that variable.

There are plenty more uses for references, i’ll explain how to use them in foreach loops in the next post.

Tell us what you think !