How Can I Differentiate Between Sequential and Associative Arrays in PHP?

Identifying Associative or Sequential Arrays in PHP

With PHP's treatment of arrays as associative by default, distinguishing between sequential and associative arrays poses a challenge. This arises from the need to categorize arrays as either "lists" (numeric keys starting from 0) or associative arrays (named keys).

Identifying Sequential Arrays

To determine if an array is sequential, the following function can be employed:

function array_is_list(array $arr)
{
    if ($arr === []) {
        return true;
    }
    return array_keys($arr) === range(0, count($arr) - 1);
}

Using this function, we can differentiate the following arrays:

$arr1 = ['apple', 'orange', 'tomato', 'carrot'];
$arr2 = ['fruit1' => 'apple', 'fruit2' => 'orange', 'veg1' => 'tomato', 'veg2' => 'carrot'];

var_dump(array_is_list($arr1)); // true (sequential array)
var_dump(array_is_list($arr2)); // false (associative array)

Legacy Code and PHP 8.1

For PHP 8.1 and above, the built-in array_is_list() function eliminates the need for the custom function:

var_dump(array_is_list(['a', 'b', 'c'])); // true
var_dump(array_is_list(['1' => 'a', '0' => 'b', '2' => 'c'])); // true
var_dump(array_is_list(['a' => 'a', 'b' => 'b', 'c' => 'c'])); // false

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