PHP references allow you to make two variables to refer to the same content. Meaning, when you do:
<?php
$a =& $b;
?>
it means that $a and $b point to the same content.
Note: $a and $b are completely equal here. $a is not pointing to $b or vice versa. $a and $b are pointing to the same place.
Note: If arrays with references are copied, their values are not dereferenced. This is valid also for arrays passed by value to functions.
Note: If you assign, pass, or return an undefined variable by reference, it will get created.
Example #1 Using references with undefined variables
<?php
function foo(&$var) { }
foo($a); // $a is "created" and assigned to null
$b = array();
foo($b['b']);
var_dump(array_key_exists('b', $b)); // bool(true)
$c = new StdClass;
foo($c->d);
var_dump(property_exists($c, 'd')); // bool(true)
?>
The same syntax can be used with functions that return references, and with the new operator (in PHP 4.0.4 and later):
<?php
$bar =& new fooclass();
$foo =& find_var($bar);
?>
Since PHP 5, new returns a reference automatically, so using =& in this context is deprecated and produces an E_STRICT message.
Note: Not using the & operator causes a copy of the object to be made. If you use $this in the class it will operate on the current instance of the class. The assignment without & will copy the instance (i.e. the object) and $this will operate on the copy, which is not always what is desired. Usually you want to have a single instance to work with, due to performance and memory consumption issues.
While you can use the @ operator to mute any errors in the constructor when using it as @new, this does not work when using the &new statement. This is a limitation of the Zend Engine and will therefore result in a parse error.
If you assign a reference to a variable declared global inside a function, the reference will be visible only inside the function. You can avoid this by using the $GLOBALS array.
Example #2 Referencing global variables inside functions
<?php
$var1 = "Example variable";
$var2 = "";
function global_references($use_globals)
{
global $var1, $var2;
if (!$use_globals) {
$var2 =& $var1; // visible only inside the function
} else {
$GLOBALS["var2"] =& $var1; // visible also in global context
}
}
global_references(false);
echo "var2 is set to '$var2'\n"; // var2 is set to ''
global_references(true);
echo "var2 is set to '$var2'\n"; // var2 is set to 'Example variable'
?>
Think about global $var; as a shortcut to $var =& $GLOBALS['var'];. Thus assigning another reference to $var only changes the local variable's reference.
Note: If you assign a value to a variable with references in a foreach statement, the references are modified too.
Example #3 References and foreach statement
<?php
$ref = 0;
$row =& $ref;
foreach (array(1, 2, 3) as $row) {
// do something
}
echo $ref; // 3 - last element of the iterated array
?>
The second thing references do is to pass variables by reference. This is done by making a local variable in a function and a variable in the calling scope referencing the same content. Example:
<?php
function foo(&$var)
{
$var++;
}
$a=5;
foo($a);
?>
will make $a to be 6. This happens because in the function foo the variable $var refers to the same content as $a. For more information on this, read the passing by reference section.
The third thing references can do is return by reference.