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

Determining the Nature of PHP Arrays: Associative or Sequential

In PHP, arrays are inherently associative, lacking built-in mechanisms to differentiate between numeric keys starting from 0 (sequential arrays) and arbitrary string keys (associative arrays). This distinction can be crucial for various programming scenarios.

Discerning Sequential and Associative Arrays

To determine if an array is sequential, you can leverage the newly introduced array_is_list() function in PHP 8.1. However, for older PHP versions, a custom function can provide a practical alternative:

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

This function checks if the array is empty or if its keys are a continuous sequence of numbers starting from 0. If both conditions are met, the array is considered sequential.

Example Usage

To illustrate the functionality of the array_is_list() function:

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

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