PHP foreach loop does not update value

Question

If the PHP program shown below is stored in bug.php, then this command

php bug.php

will produce this output when using PHP 7.0.33-0ubuntu0.16.04.1 (cli) ( NTS ) Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies with Zend OPcache v7.0.33-0ubuntu0.16.04.1, Copyright (c) 1999-2017, by Zend Technologies

Output:

Initial array
Array
(
    [0] => aa
    [1] => bb
)
j = 0  line = aa
element 0 is aa

j = 1  line = bb    <----------------------------------------------
element 1 is ***bb  <----------------------------------------------

Final array
Array
(
    [0] => aa
    [1] => ***bb
)

Notice the marked lines above. Element $A[1] is "***bb", yet $line - which the foreach should associate with key 1 - is "bb".

Is this a bug, or is there some PHP subtlety I am unaware of?

<?php

    $A = array
    (
    "aa",
    "bb",
    );

    echo "Initial array\n";
    print_r($A);

    foreach ($A as $j => $line)
    {
        echo "j = $j  line = $line\n";
        echo "element $j is {$A[$j]}\n\n";

        if ($j == 0)
            $A[1] = "***" . $A[1];
    }

    echo "Final array\n";
    print_r($A);

?>

You modified $A[1] on the first pass through the foreach so it now contained ***bb which you then display the 2nd pass through the foreach. Seems completely correct from where I'm sitting. — Dave
@Dave I don't think you understand the question and example fully. — JBES

Shlomi Hassid Shlomi Hassid · Accepted Answer · 2019-03-04T21:27:02

The array is passed to the foreach as a copy - unless you pass it as a reference (indicate that by adding a & to the $value) it will not reflect the updates of the value you are doing in the first iteration.

Consider this:

foreach ($A as $j => &$line)
{
    echo "j = $j  line = $line\n";
    echo "element $j is {$A[$j]}\n\n";

    if ($j == 0)
       $A[1] = "***" . $A[1];
}
/* Will output:

 * j = 0  line = aa
 * element 0 is aa
 *
 * j = 1  line = ***bb
 * element 1 is ***bb
 */

You see the &line that mean we are passing into the block a reference and now any change is reflected to the array and not to a $line copy of the initial array you passed into the loop block.

Hope I was clear...

I notice the docs say that exactly:

It says that exactly in documentation :)

In order to be able to directly modify array elements within the loop precede $value with &. In that case the value will be assigned by reference.

PHP foreach loop does not update value

3 Answers