foreach loop and reference of &$value

Question

Why is an empty foreach loop can change the result.

I have the following code:

$variable = [1,2,3,4];
foreach ($variable  as $key => &$value) 
  $value ++;

var_dump($variable);

The result I get is:

array (size=4)
  0 => int 2
  1 => int 3
  2 => int 4
  3 => &int 5

Now, when I add an empty foreach loop like this:

$variable  = [1,2,3,4];
foreach ($variable  as $key => &$value) 
  $value ++;

foreach ($variable  as $key => $value);

var_dump($variable);

I get this :

array (size=4)
  0 => int 2
  1 => int 3
  2 => int 4
  3 => &int 4

can someone explain me why the last element doesn't change when I add the second empty loop, and why there is a & infront of the last element?

Answer 1

At the end of the first loop, $value is pointing to the same place as $variable[3] (they are pointing to the same location in memory):

$variable  = [1,2,3,4];
foreach ($variable  as $key => &$value) 
    $value ++;

Even as this loop is finished, $value is still a reference that's pointing to the same location in memory as $variable[3], so each time you store a value in $value, this also overwrites the value stored for $variable[3]:

foreach ($variable as $key => $value);
var_dump($variable);

With each evaluation of this foreach, both $value and $variable[3] are becoming equal to the value of the iterable item in $variable.

So in the 3rd iteration of the second loop, $value and $variable[3] become equal to 4 by reference, then during the 4th and final iteration of the second loop, nothing changes because you're passing the value of $variable[3] (which is still &$value) to $value (which is still &$value).

It's very confusing, but it's not even slightly idiosyncratic; it's the code executing exactly as it should.

More info here: PHP: Passing by Reference

---

To prevent this behavior it is sufficient to add an unset($value); statement after each loop where it is used. An alternative to the unset may be to enclose the foreach loop in a self calling closure, in order to force $value to be local, but the amount of additional characters needed to do that is bigger than just unsetting it:

(function($variable){
   foreach ($variable  as $key => &$value) $value++;
})($variable);

Answer 2

This is a name collision: the name $value introduced in the first loop exists after it and is used in the second loop. So all assignments to it are in fact assignments to the original array. What you did is easier observed in this code:

  $variable = [1,2,3,4];
  foreach ($variable  as $key => &$value) 
    $value ++;
  $value = 123; // <= here you alter the array!
  var_dump($variable);

and you will see $variable[3] as 123.

One way to avoid this is, as others said, to unset ($value) after the loop, which should be a good practice as recommended by the manual. Another way is to use another variable in the second loop:

  $variable  = [1,2,3,4];
  foreach ($variable  as $key => &$value) 
    $value ++;
  foreach ($variable  as $key => $val);
  var_dump($variable);

which does not alter your array.

Answer 3

Obligatory statement: References are evil!

Stepping through your code:

$variable  = [1,2,3,4];
foreach ($variable  as $key => &$value) 
    $value++;

After the loop completes; $value is a reference to $variable[3] and thus has the value of int(4).

foreach ($variable as $key => $value);

At each iteration, $variable[3] gets assigned an element of $variable[<k>] where 0 <= k < 3. At the last iteration it gets assigned to its own value which is that of the previous iteration, so it's int(4).

Unsetting $value in between the two loops resolves the situation. See also an earlier answer by me.

Answer 4

After loop you should unset this reference using:

unset($value);

So your whole code should work like this:

  $variable  = [1,2,3,4];
  foreach ($variable  as $key => &$value) {
    $value++;
  }
  unset($value);
  var_dump($variable); 

There is no point to put unset($value); inside the loop

Explanation - after loop $value is still set to the last element of array so you can use after your loop $value = 10; (before unset) and you will see that last element of array has been changed to 10. It seems that var_dump want to help us a bit in this case and shows there is reference for last element and of course when we use unset we have desired output of var_dump.

You could also look at the following script:

<?php
  $array = [1, 2, 3, 4];

  var_dump($array);

  $x = &$array[2];

  var_dump($array);

  $x += 20;

  unset($x);

  var_dump($array);

?>

We don't use loop here and if reference is set to element of array, var_dump shows us this putting & before type of this element.

However if the above code we changed reference and set it this way $x = &$array; var_dump wouldn't show us any reference.

Also in the following code:

<?php
$x = 23;
$ref = &$x;

var_dump($x);

?>

var_dump() won't give us any hint.

Answer 5

The last element of the array will remian even after the foreach loop ..So its needed to use unset function outside the loop ..That is

$variable  = [1,2,3,4];
  foreach ($variable  as $key => &$value) {
$value++;


}
  unset($value);
  var_dump($variable); 

The link to the manual can be found here http://php.net/manual/en/control-structures.foreach.php

Answer 6

As phil stated in the comments:

As mentioned in the manual, you should unset() references after use.

---

$variable  = [1,2,3,4];

foreach ($variable  as $key => &$value)  {
  $value ++;
}
unset($value);

foreach ($variable  as $key => $value);


print_r($variable);

Will return:

Array
(
    [0] => 2
    [1] => 3
    [2] => 4
    [3] => 5
)

Example

---

Explanation

Taken from the foreach() manual. (^{See the big red box})

Reference of a $value and the last array element remain even after the foreach loop. It is recommended to destroy it by unset().

It basically means: That the referenced value &$value and the last element/item in the array, which in this case is 4 remain the same. To counter-act this issue, you'll have to unset() the value after use, otherwise it will stay in the array as its original value (if that makes sense).

You should also read this: How does PHP 'foreach' actually work?