What References Do
PHP references allow you to make two variables to refer to the
same content. Meaning, when you do:
it means that
$a and
$b
point to the same content.
Note:
$a and $b are completely
equal here, that's not $a is pointing to
$b or vice versa, that's
$a and $b pointing to the
same place.
Note:
If array with references is copied, its values are not dereferenced.
This is valid also for arrays passed by value to functions.
Note:
If you assign, pass or return an undefined variable by reference,
it will get created.
Example #1 Using references with undefined variables
<?php
function foo(&$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 new operator (in PHP 4.0.4 and later):
Since PHP 5,
new return
reference automatically so using
=& in this
context is deprecated and produces E_STRICT level 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 parser error.
Warning
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 function
<?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 other 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 reference to the same content. Example:
will make
$a to be 6. This happens because in
the function
foo the variable
$var refers to the same content as
$a. See also more detailed explanations about
passing by reference.
The third thing reference can do is return by reference.
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