PHP foreach by Reference

20 May 2015

Today, I came across a little gotcha in PHP which I find tends to rear it’s head every once and a while. The symptoms are apparent when iterating through an array using foreach. You’ll find that the penultimate element is identical to the last element and that the original last element has been removed from the array.

Here’s an example:

$people = [
    ['name' => 'Daphne', 'score' => 0],
    ['name' => 'Portia', 'score' => 0.25],
    ['name' => 'Agnus', 'score' => 0.5],
    ['name' => 'Denise', 'score' => 0.75],
    ['name' => 'Gary', 'score' => 1],
];

foreach ($people as &$person) {
    $person['score'] *= 100;
}

foreach ($people as $person) {
    printf(
        "%s = %d%%\n",
        $person['name'],
        $person['score']
    );
}

In our first iteration we’re converting each person’s score into a percentage. On the second iteration we’re outputing the data from the array. This will output:

Daphne = 0%
Portia = 25%
Agnus = 50%
Denise = 75%
Denise = 75%

Denise’s score is shown twice and Gary is not shown at all! From the outset it appears to be a bug in PHP, but upon closer inspection the issue is from referencing $person in the first iteration.

The PHP manual page for foreach warns about this:

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

During the second iteration $person is still a reference to the last element in the array, so each time we’re iterating over $people the last element is changing. When it comes to iterating over the last element in the array it’s set as the penultimate element, which is why we see it twice.

This can be seen in this example:

$people = ['Daphne', 'Portia', 'Agnus', 'Denise', 'Gary'];

foreach ($people as &$person) {
    printf(
        "[%s] %s\n",
        $person,
        implode(', ', $people)
    );
}

echo "\n";

foreach ($people as $person) {
    printf(
        "[%s] %s\n",
        $person,
        implode(', ', $people)
    );
}

During the first iteration, the $people array is unchanged, but in the second the last element as the reference of $person keeps changing:

[Daphne] Daphne, Portia, Agnus, Denise, Gary
[Portia] Daphne, Portia, Agnus, Denise, Gary
[Agnus] Daphne, Portia, Agnus, Denise, Gary
[Denise] Daphne, Portia, Agnus, Denise, Gary
[Gary] Daphne, Portia, Agnus, Denise, Gary

[Daphne] Daphne, Portia, Agnus, Denise, Daphne
[Portia] Daphne, Portia, Agnus, Denise, Portia
[Agnus] Daphne, Portia, Agnus, Denise, Agnus
[Denise] Daphne, Portia, Agnus, Denise, Denise
[Denise] Daphne, Portia, Agnus, Denise, Denise

The answer? If we unset $person after the first iteration, this removes the reference and everything works as we’d expect:

unset($person);