International PHP Conference 2019 - Spring Edition

Was Referenzen leisten

Es können grundsätzlich drei Operationen mit Referenzen durchgeführt werden: Referenzzuweisung, Referenzübergabe und Referenzrückgabe. Dieser Abschnitt enthält eine Einführung zu diesen Operationen mit Links zu weiteren Informationen.

Referenzzuweisung

PHP Referenzen erlauben es, zwei Variablennamen, sich auf den gleichen Variableninhalt beziehen zu lassen. Zum Beispiel:

<?php
$a 
=& $b
?>
Das bedeutet, dass $a und $b auf denselben Inhalt zeigen.

Hinweis:

$a und $b sind hier gleichwertig, und $a ist nicht nur ein Zeiger auf $b oder umgekehrt, sondern $a und $b zeigen auf denselben Platz.

Hinweis:

Wenn man eine undefinierte Variable per Referenz zuweist, übergibt oder zurückgibt, wird sie erstellt.

Beispiel #1 Referenzen mit undefinierten Variablen benutzen

<?php
function foo(&$var) { }

foo($a); // $a wird "erstellt" mit dem Wert 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)
?>

Diese Syntax kann auch mit Funktionen, die Referenzen zurückgeben, benutzt werden:

<?php
$foo 
=& find_var($bar);
?>
Seit PHP 5 gibt new automatisch eine Referenz zurück, so dass die Verwendung von =& in diesem Zusammenhang missbilligt ist, und von PHP 5.3 an einen E_DEPRECATED Fehler erzeugt, und einen E_STRICT Fehler in früheren Versionen. Von PHP 7.0 an ist dies syntaktisch ungültig. (Technisch gesehen ist der Unterschied, dass in PHP 5 Objektvariablen, ganz ähnlich wie Ressourcen, lediglich ein Zeiger auf die eigentlichen Objektdaten sind, so dass diese Objektreferenzen keine "Referenzen" in dem obigen Sinne (Aliase) sind. Weitere Informationen können Objekte und Referenzen entnommen werden.)

Warnung

Wird eine Referenz auf eine Variable, die in einer Funktion als global deklariert wurde, zugewiesen wird, dann ist die Referenz nur innerhalb der Funktion sichtbar. Das kann vermieden werden, indem das $GLOBALS-Array verwendet wird.

Beispiel #2 Referenzieren globaler Variablen innerhalb von Funktionen

<?php
$var1 
"Beispiel-Variable";
$var2 "";

function 
global_references($use_globals)
{
    global 
$var1$var2;
    if (!
$use_globals) {
        
$var2 =& $var1// nur innerhalb der Funktion sichtbar
    
} else {
        
$GLOBALS["var2"] =& $var1// ebenso im globalen Kontext sichtbar
    
}
}

global_references(false);
echo 
"Der Wert von var2 ist '$var2'\n"// Der Wert von var2 ist ''
global_references(true);
echo 
"Der Wert von var2 ist '$var2'\n"// Der Wert von var2 ist 'Beispiel-Variable'
?>
global $var; kann als Abkürzung für $var =& $GLOBALS['var']; angesehen werden. Daher ändert die Zuweisung einer anderen Referenz zu $var nur die Referenz der lokalen Variable.

Hinweis:

Wenn man einer Variable einen Wert per Referenz in einer foreach-Anweisung zuweist, werden auch die Referenzen geändert.

Beispiel #3 Referenzen und die foreach-Anweisung

<?php
$ref 
0;
$row =& $ref;
foreach (array(
123) as $row) {
    
// mach was
}
echo 
$ref// 3 - letztes Element des durchlaufenen Arrays
?>

Obgleich keine Referenzzuweisung im engeren Sinne, können Ausdrücke, die mit dem Sprachkonstrukt array() erstellt werden, sich ebenso verhalten, wenn dem hinzuzufügenden Array-Element & vorangestellt wird. Beispiel:

<?php
$a 
1;
$b = array(23);
$arr = array(&$a, &$b[0], &$b[1]);
$arr[0]++; $arr[1]++; $arr[2]++;
/* $a == 2, $b == array(3, 4); */
?>

Es ist allerdings zu beachten, dass Referenzen in Array potentiell gefährlich sind. Eine normale (also nicht Referenz-) Zuweisung mit einer Referenz auf der rechten Seite wandelt die linke Seite nicht in eine Referenz, aber Referenzen innerhalb von Arrays bleiben bei solchen normalen Zuweisungen erhalten. Dies gilt ebenso für Funktionsaufrufe, bei denen das Array als Wertübergabe entgegengenommen wird. Beispiel:

<?php
/* Zuweisung skalarer Variablen */
$a 1;
$b =& $a;
$c $b;
$c 7//$c ist keine Referenz; keine Änderung an $a oder $b

/* Zuweisung von Array-Variablen */
$arr = array(1);
$a =& $arr[0]; //$a und $arr[0] sind in derselben Referenzmenge
$arr2 $arr//keine Referenzzuweisung!
$arr2[0]++;
/* $a == 2, $arr == array(2) */
/* Der Inhalt von $arr hat sich geändert, obwohl es keine Referenz ist! */
?>
Anders ausgedrückt, ist das Referenzverhalten von Array auf einer Element-für-Element-Basis definiert; das Referenzverhalten einzelner Elemente ist getrennt vom Referenzzustand des Array-Behälters.

Referenzübergabe

Eine weitere Einsatzmöglichkeit von Referenzen ist die Übergabe von Parametern an eine Funktion mit pass-by-reference. Hierbei beziehen sich der lokale Variablenname als auch der Variablenname der aufrufenden Instanz auf denselben Variableninhalt:

<?php
function foo(&$var) {
    
$var++;
}

$a=5;
foo($a);
?>
Nach der Ausführung hat $a den Wert 6, da sich in der Funktion foo der Variablenname $var auf denselben Variableninhalt bezieht wie $a. Weitere Information hierzu sind dem Abschnitt Referenzübergabe zu entnehmen.

Referenzrückgabe

Daneben besteht die Möglichkeit aus Funktionen heraus Werte mit return by-reference zurückzugeben.

add a note add a note

User Contributed Notes 22 notes

up
25
ladoo at gmx dot at
13 years ago
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
?>
up
11
Hlavac
11 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!
up
13
elrah [] polyptych [dot] com
7 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";
?>
up
5
nay at woodcraftsrus dot com
7 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 );
?>
up
3
Oddant
5 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)

?>
up
4
Amaroq
8 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.
?>
up
2
php.devel at homelinkcs dot com
13 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.
up
3
amp at gmx dot info
11 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)
up
1
php at hood dot id dot au
11 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)
up
1
Amaroq
10 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"
?>
up
1
Drewseph
10 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.
up
0
charles at org oo dot com
10 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);
?>
up
0
dovbysh at gmail dot com
11 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

?>
up
0
joachim at lous dot org
15 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.
up
-1
akinaslan at gmail dot com
7 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 $reference->c; //Echoes 2.
?>
up
-1
dnhuff at acm dot org
10 years ago
In reply to Drewseph using foo($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.
up
-1
firespade at gmail dot com
11 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
up
-2
Amaroq
8 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.
up
-1
Anonymous
2 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'...
up
-4
strata_ranger at hotmail dot com
9 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')));

?>
up
-4
butshuti at smartrwanda dot org
5 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)!
up
-4
admin at torntech dot com
5 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 ,
To Top