I ran into something when using an expanded version of the example of pbaltz at NO_SPAM dot cs dot NO_SPAM dot wisc dot edu below.
This could be somewhat confusing although it is perfectly clear if you have read the manual carfully. It makes the fact that references always point to the content of a variable perfectly clear (at least to me).
<?php
$a = 1;
$c = 2;
$b =& $a; // $b points to 1
$a =& $c; // $a points now to 2, but $b still to 1;
echo $a, " ", $b;
// Output: 2 1
?>
Que font les références ?
Il existe trois principales utilisations des références : l'assignation par référence, le passage par référence et le retour par référence. Cette section va introduire ces opérations, avec des liens vers plus de précisions.
Assignation par référence
Dans ce premier cas, les références PHP permettent de faire en sorte que deux variables référencent le même contenu. Par exemple :
<?php
$a =& $b;
?>Note:
$a et $b sont complètement égales ici : ce n'est pas $a qui pointe sur $b, ou vice-versa. Ce sont bien $a et $b qui pointent sur le même contenu.
Note:
Si vous assignez, passez, ou retournez, une variable indéfinie par référence, elle sera créée automatiquement.
Exemple #1 Utilisation des références avec des variables indéfinies
<?php
function foo(&$var) { }
foo($a); // $a est "créée" et assignée à 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)
?>
La même syntaxe peut être utilisée avec les fonctions qui retournent des références :
<?php
$foo =& find_var($bar);
?>Utiliser la même syntaxe avec une fonction qui ne retourne par référence génèrera une erreur, comme l'utiliser avec le résultat de l'opérateur new. Tant bien que les objets sont passés comme des pointeurs, ceci n'est pas identique aux références comme expliqué dans la section les Objets et références.
Avertissement
Voyez
Si vous assignez une référence à une variable déclarée comme global
dans une fonction, la référence ne sera visible qu'au sein de la fonction.
Vous pouvez éviter cela en utilisant le tableau $GLOBALS.
Exemple #2 Référencer des variables globales depuis des fonctions
<?php
$var1 = "Variable Exemple";
$var2 = "";
function global_references($use_globals)
{
global $var1, $var2;
if (!$use_globals) {
$var2 =& $var1; // visible uniquement dans la fonction
} else {
$GLOBALS["var2"] =& $var1; // visible également dans le contexte global
}
}
global_references(false);
echo "var2 est défini à '$var2'\n"; // var2 est défini à ''
global_references(true);
echo "var2 est défini à '$var2'\n"; // var2 est défini à 'Variable Exemple'
?>global $var; comme un raccourci pour $var
=& $GLOBALS['var'];. De ce fait, assigner une autre référence à
$var modifie uniquement la référence locale de la variable.
Note:
Si vous assignez une valeur à une variable qui a des références dans une structure foreach, les références seront également modifiées.
Exemple #3 Références et structure foreach
<?php
$ref = 0;
$row =& $ref;
foreach (array(1, 2, 3) as $row) {
// faites quelque chose
}
echo $ref; // 3 - le dernier élément du tableau itéré
?>
Même si ce n'est pas strictement une assignation par référence, les expressions
créées avec la structure de langage
array() peuvent aussi
se comporter comme telles, en préfixant par & l'élément du tableau.
Voici un exemple :
<?php
$a = 1;
$b = array(2, 3);
$arr = array(&$a, &$b[0], &$b[1]);
$arr[0]++; $arr[1]++; $arr[2]++;
/* $a == 2, $b == array(3, 4); */
?>Notez par contre que les références à l'intérieur des tableaux peuvent s'avérer dangereuses. Utiliser une assignation normale (pas par référence) avec une référence à droite de l'opérateur ne transforme pas la partie gauche de l'assignation en référence, mais les références à l'intérieur des tableaux sont préservées. Ceci s'applique également aux appels de fonctions avec un tableau passé par valeur. Exemple :
<?php
/* Assignation de variables scalaires */
$a = 1;
$b =& $a;
$c = $b;
$c = 7; // $c n'est pas une référence ; pas de changement à $a ou $b
/* Assignation de variables de type tableau */
$arr = array(1);
$a =& $arr[0]; // $a et $arr[0] sont des références vers la même valeur
$arr2 = $arr; // PAS une assignation par référence!
$arr2[0]++;
/* $a == 2, $arr == array(2) */
/* Les contenus de $arr sont changés même si ce n'était pas une référence ! */
?>Passage par référence
Le deuxième intérêt des références est de permettre de passer des variables par référence. On réalise ceci en faisant référencer le même contenu par une variable locale à un fonction et par une variable du contexte appelant. Par exemple :
<?php
function foo(&$var) {
$var++;
}
$a=5;
foo($a);
?>Retour par référence
Le troisième intérêt des références est de permettre le retour de valeurs par référence.
add a note
User Contributed Notes 23 notes
ladoo at gmx dot at ¶
19 years ago
Hlavac ¶
17 years ago
Watch out for this:
foreach ($somearray as &$i) {
// update some $i...
}
...
foreach ($somearray as $i) {
// last element of $somearray is mysteriously overwritten!
}
Problem is $i contians reference to last element of $somearray after the first foreach, and the second foreach happily assigns to it!
elrah [] polyptych [dot] com ¶
13 years ago
It appears that references can have side-effects. Below are two examples. Both are simply copying one array to another. In the second example, a reference is made to a value in the first array before the copy. In the first example the value at index 0 points to two separate memory locations. In the second example, the value at index 0 points to the same memory location.
I won't say this is a bug, because I don't know what the designed behavior of PHP is, but I don't think ANY developers would expect this behavior, so look out.
An example of where this could cause problems is if you do an array copy in a script and expect on type of behavior, but then later add a reference to a value in the array earlier in the script, and then find that the array copy behavior has unexpectedly changed.
<?php
// Example one
$arr1 = array(1);
echo "\nbefore:\n";
echo "\$arr1[0] == {$arr1[0]}\n";
$arr2 = $arr1;
$arr2[0]++;
echo "\nafter:\n";
echo "\$arr1[0] == {$arr1[0]}\n";
echo "\$arr2[0] == {$arr2[0]}\n";
// Example two
$arr3 = array(1);
$a =& $arr3[0];
echo "\nbefore:\n";
echo "\$a == $a\n";
echo "\$arr3[0] == {$arr3[0]}\n";
$arr4 = $arr3;
$arr4[0]++;
echo "\nafter:\n";
echo "\$a == $a\n";
echo "\$arr3[0] == {$arr3[0]}\n";
echo "\$arr4[0] == {$arr4[0]}\n";
?>
amp at gmx dot info ¶
17 years ago
Something that might not be obvious on the first look:
If you want to cycle through an array with references, you must not use a simple value assigning foreach control structure. You have to use an extended key-value assigning foreach or a for control structure.
A simple value assigning foreach control structure produces a copy of an object or value. The following code
$v1=0;
$arrV=array(&$v1,&$v1);
foreach ($arrV as $v)
{
$v1++;
echo $v."\n";
}
yields
0
1
which means $v in foreach is not a reference to $v1 but a copy of the object the actual element in the array was referencing to.
The codes
$v1=0;
$arrV=array(&$v1,&$v1);
foreach ($arrV as $k=>$v)
{
$v1++;
echo $arrV[$k]."\n";
}
and
$v1=0;
$arrV=array(&$v1,&$v1);
$c=count($arrV);
for ($i=0; $i<$c;$i++)
{
$v1++;
echo $arrV[$i]."\n";
}
both yield
1
2
and therefor cycle through the original objects (both $v1), which is, in terms of our aim, what we have been looking for.
(tested with php 4.1.3)
Anonymous ¶
9 years ago
to reply to ' elrah [] polyptych [dot] com ', one thing to keep in mind is that array (or similar large data holders) are by default passed by reference. So the behaviour is not side effect. And for array copy and passing array inside function always done by 'pass by reference'...
nay at woodcraftsrus dot com ¶
13 years ago
in PHP you don't really need pointer anymore if you want to share an object across your program
<?php
class foo{
protected $name;
function __construct($str){
$this->name = $str;
}
function __toString(){
return 'my name is "'. $this->name .'" and I live in "' . __CLASS__ . '".' . "\n";
}
function setName($str){
$this->name = $str;
}
}
class MasterOne{
protected $foo;
function __construct($f){
$this->foo = $f;
}
function __toString(){
return 'Master: ' . __CLASS__ . ' | foo: ' . $this->foo . "\n";
}
function setFooName($str){
$this->foo->setName( $str );
}
}
class MasterTwo{
protected $foo;
function __construct($f){
$this->foo = $f;
}
function __toString(){
return 'Master: ' . __CLASS__ . ' | foo: ' . $this->foo . "\n";
}
function setFooName($str){
$this->foo->setName( $str );
}
}
$bar = new foo('bar');
print("\n");
print("Only Created \$bar and printing \$bar\n");
print( $bar );
print("\n");
print("Now \$baz is referenced to \$bar and printing \$bar and \$baz\n");
$baz =& $bar;
print( $bar );
print("\n");
print("Now Creating MasterOne and Two and passing \$bar to both constructors\n");
$m1 = new MasterOne( $bar );
$m2 = new MasterTwo( $bar );
print( $m1 );
print( $m2 );
print("\n");
print("Now changing value of \$bar and printing \$bar and \$baz\n");
$bar->setName('baz');
print( $bar );
print( $baz );
print("\n");
print("Now printing again MasterOne and Two\n");
print( $m1 );
print( $m2 );
print("\n");
print("Now changing MasterTwo's foo name and printing again MasterOne and Two\n");
$m2->setFooName( 'MasterTwo\'s Foo' );
print( $m1 );
print( $m2 );
print("Also printing \$bar and \$baz\n");
print( $bar );
print( $baz );
?>
Oddant ¶
11 years ago
About the example on array references.
I think this should be written in the array chapter as well.
Indeed if you are new to programming language in some way, you should beware that arrays are pointers to a vector of Byte(s).
<?php $arr = array(1); ?>
$arr here contains a reference to which the array is located.
Writing :
<?php echo $arr[0]; ?>
dereferences the array to access its very first element.
Now something that you should also be aware of (even you are not new to programming languages) is that PHP use references to contains the different values of an array. And that makes sense because the type of the elements of a PHP array can be different.
Consider the following example :
<?php
$arr = array(1, 'test');
$point_to_test =& $arr[1];
$new_ref = 'new';
$arr[1] =& $new_ref;
echo $arr[1]; // echo 'new';
echo $point_to_test; // echo 'test' ! (still pointed somewhere in the memory)
?>
dovbysh at gmail dot com ¶
17 years ago
Solution to post "php at hood dot id dot au 04-Mar-2007 10:56":
<?php
$a1 = array('a'=>'a');
$a2 = array('a'=>'b');
foreach ($a1 as $k=>&$v)
$v = 'x';
echo $a1['a']; // will echo x
unset($GLOBALS['v']);
foreach ($a2 as $k=>$v)
{}
echo $a1['a']; // will echo x
?>
charles at org oo dot com ¶
17 years ago
points to post below me.
When you're doing the references with loops, you need to unset($var).
for example
<?php
foreach($var as &$value)
{
...
}
unset($value);
?>
php.devel at homelinkcs dot com ¶
20 years ago
In reply to lars at riisgaardribe dot dk,
When a variable is copied, a reference is used internally until the copy is modified. Therefore you shouldn't use references at all in your situation as it doesn't save any memory usage and increases the chance of logic bugs, as you discoved.
Amaroq ¶
14 years ago
I think a correction to my last post is in order.
When there is a constructor, the strange behavior mentioned in my last post doesn't occur. My guess is that php was treating reftest() as a constructor (maybe because it was the first function?) and running it upon instantiation.
<?php
class reftest
{
public $a = 1;
public $c = 1;
public function __construct()
{
return 0;
}
public function reftest()
{
$b =& $this->a;
$b++;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
}
$reference = new reftest();
$reference->reftest();
$reference->reftest2();
echo $reference->a; //Echoes 2.
echo $reference->c; //Echoes 2.
?>
akinaslan at gmail dot com ¶
13 years ago
In this example class name is different from its first function and however there is no construction function. In the end as you guess "a" and "c" are equal. So if there is no construction function at same time class and its first function names are the same, "a" and "c" doesn't equal forever. In my opinion php doesn't seek any function for the construction as long as their names differ from each others.
<?php
class reftest_new
{
public $a = 1;
public $c = 1;
public function reftest()
{
$b =& $this->a;
$b++;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
}
$reference = new reftest_new();
$reference->reftest();
$reference->reftest2();
echo $reference->a; //Echoes 2.
echo $reference->c; //Echoes 2.
?>
dexant9t at gmail dot com ¶
3 years ago
It matters if you are playing with a reference or with a value
Here we are working with values so working on a reference updates original variable too;
$a = 1;
$c = 22;
$b = & $a;
echo "$a, $b"; //Output: 1, 1
$b++;
echo "$a, $b";//Output: 2, 2 both values are updated
$b = 10;
echo "$a, $b";//Output: 10, 10 both values are updated
$b =$c; //This assigns value 2 to $b which also updates $a
echo "$a, $b";//Output: 22, 22
But, if instead of $b=$c you do
$b = &$c; //Only value of $b is updated, $a still points to 10, $b serves now reference to variable $c
echo "$a, $b"//Output: 10, 22
Amaroq ¶
16 years ago
The order in which you reference your variables matters.
<?php
$a1 = "One";
$a2 = "Two";
$b1 = "Three";
$b2 = "Four";
$b1 =& $a1;
$a2 =& $b2;
echo $a1; //Echoes "One"
echo $b1; //Echoes "One"
echo $a2; //Echoes "Four"
echo $b2; //Echoes "Four"
?>
Drewseph ¶
16 years ago
If you set a variable before passing it to a function that takes a variable as a reference, it is much harder (if not impossible) to edit the variable within the function.
Example:
<?php
function foo(&$bar) {
$bar = "hello\n";
}
foo($unset);
echo($unset);
foo($set = "set\n");
echo($set);
?>
Output:
hello
set
It baffles me, but there you have it.
dnhuff at acm dot org ¶
16 years ago
In reply to Drewseph using foo($a = 'set'); where $a is a reference formal parameter.
$a = 'set' is an expression. Expressions cannot be passed by reference, don't you just hate that, I do. If you turn on error reporting for E_NOTICE, you will be told about it.
Resolution: $a = 'set'; foo($a); this does what you want.
firespade at gmail dot com ¶
17 years ago
Here's a good little example of referencing. It was the best way for me to understand, hopefully it can help others.
$b = 2;
$a =& $b;
$c = $a;
echo $c;
// Then... $c = 2
Amaroq ¶
14 years ago
When using references in a class, you can reference $this-> variables.
<?php
class reftest
{
public $a = 1;
public $c = 1;
public function reftest()
{
$b =& $this->a;
$b = 2;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
}
$reference = new reftest();
$reference->reftest();
$reference->reftest2();
echo $reference->a; //Echoes 2.
echo $reference->c; //Echoes 2.
?>
However, this doesn't appear to be completely trustworthy. In some cases, it can act strangely.
<?php
class reftest
{
public $a = 1;
public $c = 1;
public function reftest()
{
$b =& $this->a;
$b++;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
}
$reference = new reftest();
$reference->reftest();
$reference->reftest2();
echo $reference->a; //Echoes 3.
echo $reference->c; //Echoes 2.
?>
In this second code block, I've changed reftest() so that $b increments instead of just gets changed to 2. Somehow, it winds up equaling 3 instead of 2 as it should.
php at hood dot id dot au ¶
17 years ago
I discovered something today using references in a foreach
<?php
$a1 = array('a'=>'a');
$a2 = array('a'=>'b');
foreach ($a1 as $k=>&$v)
$v = 'x';
echo $a1['a']; // will echo x
foreach ($a2 as $k=>$v)
{}
echo $a1['a']; // will echo b (!)
?>
After reading the manual this looks like it is meant to happen. But it confused me for a few days!
(The solution I used was to turn the second foreach into a reference too)
strata_ranger at hotmail dot com ¶
15 years ago
An interesting if offbeat use for references: Creating an array with an arbitrary number of dimensions.
For example, a function that takes the result set from a database and produces a multidimensional array keyed according to one (or more) columns, which might be useful if you want your result set to be accessible in a hierarchial manner, or even if you just want your results keyed by the values of each row's primary/unique key fields.
<?php
function array_key_by($data, $keys, $dupl = false)
/*
* $data - Multidimensional array to be keyed
* $keys - List containing the index/key(s) to use.
* $dupl - How to handle rows containing the same values. TRUE stores it as an Array, FALSE overwrites the previous row.
*
* Returns a multidimensional array indexed by $keys, or NULL if error.
* The number of dimensions is equal to the number of $keys provided (+1 if $dupl=TRUE).
*/
{
// Sanity check
if (!is_array($data)) return null;
// Allow passing single key as a scalar
if (is_string($keys) or is_integer($keys)) $keys = Array($keys);
elseif (!is_array($keys)) return null;
// Our output array
$out = Array();
// Loop through each row of our input $data
foreach($data as $cx => $row) if (is_array($row))
{
// Loop through our $keys
foreach($keys as $key)
{
$value = $row[$key];
if (!isset($last)) // First $key only
{
if (!isset($out[$value])) $out[$value] = Array();
$last =& $out; // Bind $last to $out
}
else // Second and subsequent $key....
{
if (!isset($last[$value])) $last[$value] = Array();
}
// Bind $last to one dimension 'deeper'.
// First lap: was &$out, now &$out[...]
// Second lap: was &$out[...], now &$out[...][...]
// Third lap: was &$out[...][...], now &$out[...][...][...]
// (etc.)
$last =& $last[$value];
}
if (isset($last))
{
// At this point, copy the $row into our output array
if ($dupl) $last[$cx] = $row; // Keep previous
else $last = $row; // Overwrite previous
}
unset($last); // Break the reference
}
else return NULL;
// Done
return $out;
}
// A sample result set to test the function with
$data = Array(Array('name' => 'row 1', 'foo' => 'foo_a', 'bar' => 'bar_a', 'baz' => 'baz_a'),
Array('name' => 'row 2', 'foo' => 'foo_a', 'bar' => 'bar_a', 'baz' => 'baz_b'),
Array('name' => 'row 3', 'foo' => 'foo_a', 'bar' => 'bar_b', 'baz' => 'baz_c'),
Array('name' => 'row 4', 'foo' => 'foo_b', 'bar' => 'bar_c', 'baz' => 'baz_d')
);
// First, let's key it by one column (result: two-dimensional array)
print_r(array_key_by($data, 'baz'));
// Or, key it by two columns (result: 3-dimensional array)
print_r(array_key_by($data, Array('baz', 'bar')));
// We could also key it by three columns (result: 4-dimensional array)
print_r(array_key_by($data, Array('baz', 'bar', 'foo')));
?>
joachim at lous dot org ¶
21 years ago
So to make a by-reference setter function, you need to specify reference semantics _both_ in the parameter list _and_ the assignment, like this:
class foo{
var $bar;
function setBar(&$newBar){
$this->bar =& newBar;
}
}
Forget any of the two '&'s, and $foo->bar will end up being a copy after the call to setBar.
butshuti at smartrwanda dot org ¶
11 years ago
This appears to be the hidden behavior: When a class function has the same name as the class, it seems to be implicitly called when an object of the class is created.
For instance, you may take a look at the naming of the function "reftest()": it is in the class "reftest". The behavior can be tested as follows:
<?php
class reftest
{
public $a = 1;
public $c = 1;
public function reftest1()
{
$b =& $this->a;
$b++;
}
public function reftest2()
{
$d =& $this->c;
$d++;
}
public function reftest()
{
echo "REFTEST() called here!\n";
}
}
$reference = new reftest();
/*You must notice the above will also implicitly call reference->reftest()*/
$reference->reftest1();
$reference->reftest2();
echo $reference->a."\n"; //Echoes 2, not 3 as previously noticed.
echo $reference->c."\n"; //Echoes 2.
?>
The above outputs:
REFTEST() called here!
2
2
Notice that reftest() appears to be called (though no explicit call to it was made)!
admin at torntech dot com ¶
11 years ago
Something that has not been discussed so far is reference of a reference.
I needed a quick and dirty method of aliasing incorrect naming until a proper rewrite could be done.
Hope this saves someone else the time of testing since it was not covered in the Does/Are/Are Not pages.
Far from best practice, but it worked.
<?php
$a = 0;
$b =& $a;
$a =& $b;
$a = 5;
echo $a . ', ' . $b;
//ouputs: 5,5
echo ' | ';
$b = 6;
echo $a . ',' . $b;
//outputs: 6,6
echo ' | ';
unset( $a );
echo $a . ', ' . $b;
//outputs: , 6
class Product {
public $id;
private $productid;
public function __construct( $id = null ) {
$this->id =& $this->productid;
$this->productid =& $this->id;
$this->id = $id;
}
public function getProductId() {
return $this->productid;
}
}
echo ' | ';
$Product = new Product( 1 );
echo $Product->id . ', ' . $Product->getProductId();
//outputs 1, 1
$Product->id = 2;
echo ' | ';
echo $Product->id . ', ' . $Product->getProductId();
//outputs 2, 2
$Product->id = null;
echo ' | ';
echo $Product->id . ', ' . $Product->getProductId();
//outouts ,
