PHP Pointers behaving unexpectedly

user_8176262 0
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While working on a project which checks the if Laravel models are related to each other I noticed some (weird?) pointer behavior going on with PHP. Below is a minimal example to reproduce what I found.

<?php

$arr = ['a', 'b', ['c']];


foreach($arr as &$letter) {
    if (!is_array($letter)) {
        $letter = [$letter];    
    }
}

dump($arr);

foreach($arr as $letter) {
    dump($arr);
}

function dump(...$dump) {
    echo '<pre>';
    var_dump($dump);
    echo '</pre>';  
}

At first I expected the dumps in this response to all return the same data:

[ ['a'], ['b'], ['c'] ]

But that is not what happened, I got the following responses:

[ ['a'], ['b'], ['c'] ]
[ ['a'], ['b'], ['a'] ]
[ ['a'], ['b'], ['b'] ]
[ ['a'], ['b'], ['b'] ]

A running example can be found here.

Why do the pointers act this way? How can I update $letter in the first loop without having to do $arr[$key] = $letter?

Edit: As people seem to be misunderstanding why there is a second foreach loop, this is to show that the array is changing without being reassigned

pointers php
2 Answers

According to the PHP documentation:

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

$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
    $value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)

// Without an `unset($value)`, `$value` is still a reference to the last item: `$arr[3]`

foreach ($arr as $key => $value) {
    // $arr[3] will be updated with each value from $arr...
    echo "{$key} => {$value} ";
    print_r($arr);
}
// ...until ultimately the second-to-last value is copied onto the last value

/* output:
   0 => 2 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 2 )
   1 => 4 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 4 )
   2 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
   3 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 ) */
user_20908530

First of all: PHP doesn't have pointers, it has references. See What references are and What references are not for more information.

The reason this happens is that $letter after the foreach loop still holds a reference to the last element of your array (which is [c]). So in your second loop, you're overriding not only $letter while looping but also the reference it points to.

To solve the problem you need to unset($letter) after your first loop:

$arr = ['a', 'b', ['c']];

foreach($arr as &$letter) {
    if (!is_array($letter)) {
        $letter = [$letter];    
    }
}
unset($letter);   // this is important

dump($arr);

foreach($arr as $letter) {
    dump($arr);
}

function dump(...$dump) {
    echo '<pre>';
    var_dump($dump);
    echo '</pre>';  
}
user_46816540
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