Zymic

What are references? References are a tool in PHP that we can use to access the content of a variable through several identifiers ("names"). When a reference is modified, the content it points to will be changed directly, this means that when editing a reference or the original variable we change it for everything referencing that content simultaneously. To put it into context, say you had set up a company, and you applied for a P.O. Box for your office. You would then have two addresses, which point to the same place. References are like the P.O. box, they are "addresses" through which we can modify the content of a variable. In terms of code we could have a variable called $var and then make a reference to it called $vartwo, when $var is edited, $vartwo also changes and vica versa. References are created using "reference binding" this is very similar to assignment, but uses =& (equals ampersand) to tell the PHP parser that we are creating a reference and not a new variable. Below is an example of creating and using references to manipulate variables:

<?php
// Create a normal variable.
$var = 5;
// Make a reference to $var
$vartwo =& $var;
// Increment $vartwo
$vartwo++;
// Output $var
echo $var;
// Outputs "6"
?>

When creating references, it is important to remember that you can only create references to variables, and not to content. This is because until the content is placed inside a variable, it has no address. Taking our previous metaphor, this would be like trying to get mail sent to you as "Joe Bloggs, no fixed address" it can't be done because no one knows where you are! Therefore, the following code is invalid:

<?php
// Try to create a new reference:
$reference =& 'text string';
?>

This will in fact, return a syntax error like "Parse error: syntax error, unexpected T_CONSTANT_ENCAPSED_STRING, expecting T_NEW or T_STRING or T_VARIABLE or '$'", because PHP cannot make a reference to unassigned values. It would be valid however if we first assigned 'text string' to a variable, and made a reference to that variable.

The most common use for references is passing by reference in functions. Using references for this means we don't need to reassign the variable, instead we modify the variable directly. Below are examples of using a function to modify a variable, one uses references, while the other uses conventional reassignment:

<?php
// Normal reassignment:
 
// int increment(int $int)
//
// Increments a variable and returns the new value.
function increment($int)
{
   return ++$int;
}
 
$var = 2;
$var = increment($var); // $var is now 3
increment($var); // No change to $var
 
// Using references:
 
// void incrementByRef(int &$int)
//
// Increments a variable and changes the value directly.
function incrementByRef(&$int)
{
   $int++;
}
 
incrementByRef($var); // $var is now 4
$var = incrementByRef($var); // Incorrect usage, sets $var to null
?>

As you can see from that example, references meant that you could call incrementByRef() directly on the variable, and change it without reassigning with "$var =", making the code required shorter.

It is important to note that unset(), when used on references will not delete the variable content, just that reference to the content. This means that if we unset() one reference or the original, we can still access the information through the other references. We can even unset the original variable, and the PHP engine will preserve the content for the other references. Like so:

<?php
// Set some variables and references:
$a = 7;
$b =& $a;
$c =& $a;
 
echo $a,$b,$c; // Outputs "777"
 
unset($a);
 
echo $a,$b,$c; // Outputs "77"
 
unset($b);
 
echo $a,$b,$c; // Outputs "7"
 
?>

In that example, both one of the references and the original variable were unset(), but the content can still be accessed through $c

More information can be found at php.net.