How to Determine if a PHP Array is Associative or Sequential

In PHP, arrays are flexible and can act as both indexed arrays (or “sequential” arrays) and associative arrays. However, PHP treats all arrays as associative by default. So, how can you differentiate between an associative array and a sequential array?

An associative array uses string keys, while a sequential array uses numeric keys starting from 0 and increasing sequentially. In this blog post, we’ll explore a few different ways to check whether a given array is associative or sequential, using practical examples and approaches that avoid expensive operations.

1. Understanding Sequential vs. Associative Arrays

Before we jump into the code, let’s clarify the difference between a sequential and an associative array.

• Sequential Array: Contains only numeric keys starting from 0 and increments by 1.

  $sequentialArray = ['apple', 'orange', 'banana', 'grape'];
  

• Associative Array: Contains string keys or non-consecutive numeric keys.

  $assocArray = ['fruit1' => 'apple', 'fruit2' => 'orange', 'veg1' => 'carrot'];
  

2. Using array_keys() and range() for Sequential Check

One of the most efficient ways to check if an array is sequential is to compare its keys to a range of consecutive integers. If the array’s keys match a sequence from 0 to count($array) - 1, it’s a sequential array.

Example:

function is_sequential(array $arr): bool {
    return array_keys($arr) === range(0, count($arr) - 1);
}

$sequentialArray = ['apple', 'orange', 'banana', 'grape'];
$assocArray = ['fruit1' => 'apple', 'fruit2' => 'orange', 'veg1' => 'tomato'];

echo is_sequential($sequentialArray) ? "Sequential" : "Associative";  // Output: Sequential
echo is_sequential($assocArray) ? "Sequential" : "Associative";  // Output: Associative

In this function, array_keys() retrieves all keys from the array, and range(0, count($arr) - 1) generates a list of numbers starting from 0. If the two arrays match, the original array is sequential.

3. Using a Simple Loop to Check for String Keys

Another approach is to iterate through the array keys and check if any key is a string. If a string key is found, the array is associative.

Example:

function is_associative(array $arr): bool {
    foreach ($arr as $key => $value) {
        if (is_string($key)) {
            return true;  // Found a string key, it's associative.
        }
    }
    return false;  // No string keys, it's sequential.
}

$sequentialArray = ['apple', 'orange', 'banana', 'grape'];
$assocArray = ['fruit1' => 'apple', 'fruit2' => 'orange'];

echo is_associative($sequentialArray) ? "Associative" : "Sequential";  // Output: Sequential
echo is_associative($assocArray) ? "Associative" : "Sequential";  // Output: Associative

This method loops through each key in the array, checking if it’s a string. If a string key is found, it immediately returns true indicating that the array is associative.

4. Using array_values() Comparison

You can also check if an array is sequential by comparing it with its own values. This method works because array_values() returns an array with re-indexed values starting from 0, which is what a sequential array should look like.

Example:

function is_sequential_by_values(array $arr): bool {
    return $arr === array_values($arr);
}

$sequentialArray = ['apple', 'orange', 'banana', 'grape'];
$assocArray = ['fruit1' => 'apple', 'fruit2' => 'orange'];

echo is_sequential_by_values($sequentialArray) ? "Sequential" : "Associative";  // Output: Sequential
echo is_sequential_by_values($assocArray) ? "Sequential" : "Associative";  // Output: Associative

In this approach, array_values() removes any custom keys and creates a sequentially indexed array. If the original array matches this, it’s a sequential array.

5. Using PHP 8.1’s array_is_list() (New Function)

With the release of PHP 8.1, there’s now a native function array_is_list() that checks if an array is a list (sequential). This function simplifies the process and reduces the need for custom functions.

Example:

$listArray = ['apple', 'orange', 'banana'];
$assocArray = ['fruit1' => 'apple', 'fruit2' => 'orange'];

echo array_is_list($listArray) ? "Sequential" : "Associative";  // Output: Sequential
echo array_is_list($assocArray) ? "Sequential" : "Associative";  // Output: Associative

This built-in function is the most efficient method in modern PHP versions for checking if an array is sequential.

In PHP, distinguishing between sequential and associative arrays is important when building dynamic applications or handling data. With multiple methods available — from manual iteration to built-in functions in PHP 8.1 — you can choose the one that best fits your use case. Whether you use array_keys(), array_values(), or the native array_is_list(), understanding how PHP handles arrays will help you write more robust and efficient code.