PHP 8.1: New array_is_list function

PHP 8.1 brings a new function array_is_list, that returns whether a given array is an array with all sequential integers starting from 0.

In other words, this function returns true if the given array is semantic list of values; an array with all keys are integers, keys start from 0, with no gaps in between.

array_is_list function returns true on empty arrays as well.

array_is_list([]); // true
array_is_list([1, 2, 3]); // true
array_is_list(['apple', 2, 3]); // true
array_is_list(['apple', 'orange']); // true
array_is_list([0 => 'apple', 'orange']); // true
array_is_list([0 => 'apple', 1 => 'orange']); // true

Any array with keys not starting from zero, or any array with not all keys are integers in sequential order evaluates to false:

// Key does not start with 0
array_is_list([1 => 'apple', 'orange']); // false

// Keys are not in order
array_is_list([1 => 'apple', 0 => 'orange']); // false

// Non-integer keys
array_is_list([0 => 'apple', 'foo' => 'bar']); false

// Non-sequential keys
array_is_list([0 => 'apple', 2 => 'bar']); false

array_is_list Function Synopsis

function array_is_list(array $array): bool {
}

• array_is_list only accepts array parameters. Passing any other type will throw a TypeError exception.
• array_is_list function does not accept iterable or other array-like class objects such as ArrayAccess, SPLFixedArray, etc.
• array_is_list is declared in the global namespace.

Polyfill

array_is_list function can be trivially polyfilled with user-land PHP code:

function array_is_list(array $array): bool {
    if (empty($array)) {
        return true;
    }

    $current_key = 0;
    foreach ($array as $key => $noop) {
        if ($key !== $current_key) {
            return false;
        }
        ++$current_key;
    }

    return true;
}

Backwards Compatibility Impact

array_is_list is a new function added in PHP 8.1. Unless there is an existing function with the same name in global namespace, there should be no backwards-compatibility issues.

Further, array_is_list function can be back-ported to almost any PHP version.

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