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PHP Using foreach


Example

Direct loop

foreach ($colors as $color) {
    echo "I am the color $color<br>";
}

Loop with keys

$foods = ['healthy' => 'Apples', 'bad' => 'Ice Cream'];
foreach ($foods as $key => $food) {
    echo "Eating $food is $key";
}

Loop by reference

In the foreach loops in the above examples, modifying the value ($color or $food) directly doesn't change its value in the array. The & operator is required so that the value is a reference pointer to the element in the array.

$years = [2001, 2002, 3, 4];
foreach ($years as &$year) {
    if ($year < 2000) $year += 2000;
}

This is similar to:

$years = [2001, 2002, 3, 4];
for($i = 0; $i < count($years); $i++) { // these two lines
    $year = &$years[$i];                // are changed to foreach by reference
    if($year < 2000) $year += 2000;
}

Concurrency

PHP arrays can be modified in any ways during iteration without concurrency problems (unlike e.g. Java Lists). If the array is iterated by reference, later iterations will be affected by changes to the array. Otherwise, the changes to the array will not affect later iterations (as if you are iterating a copy of the array instead). Compare looping by value:

$array = [0 => 1, 2 => 3, 4 => 5, 6 => 7];
foreach ($array as $key => $value) {
    if ($key === 0) {
        $array[6] = 17;
        unset($array[4]);
    }
    echo "$key => $value\n";
}

Output:

0 => 1
2 => 3
4 => 5
6 => 7

But if the array is iterated with reference,

$array = [0 => 1, 2 => 3, 4 => 5, 6 => 7];
foreach ($array as $key => &$value) {
    if ($key === 0) {
        $array[6] = 17;
        unset($array[4]);
    }
    echo "$key => $value\n";
}

Output:

0 => 1
2 => 3
6 => 17

The key-value set of 4 => 5 is no longer iterated, and 6 => 7 is changed to 6 => 17.