How to Flatten Multidimentional Array in PHP ?

Given a Multi-dimensional array, the task is to flatten the multidimensional array into a simple array (1-dimensional array) in PHP.

Examples:

Input: arr = [[1, 2], [3, 4, 5], 6]
Output: [1, 2, 3, 4, 5, 6]

Input: arr = [[1, 4, 5], [6, 7, 8]]
Output: [1, 4, 5, 6, 7, 8]

Working with multidimensional arrays is a common task in PHP. However, there are situations where you might need to flatten a multidimensional array, converting it into a single-dimensional array.

There are different approaches to flattening a multi-dimensional array, these are:

Approach 1: Using Recursive Method

The recursive approach involves traversing the nested arrays using recursion. It’s a clean and flexible method that works well for arrays of any depth.

Example:

PHP

<?php

function flattenArray($arr) {
    $res = [];

    foreach ($arr as $val) {
        if (is_array($val)) {
            $res = array_merge($res,
                flattenArray($val));
        } else {
            $res[] = $val;
        }
    }

    return $res;
}

// Driver code
$arr = [1, [2, 3, [4, 5]], 6];
$flatArr = flattenArray($arr);

print_r($flatArr);

?>

Output

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 6
)

Approach 2: Iterative Method using Stack

This approach uses an iterative method with a stack to flatten the array. It provides an alternative to recursion.

Example:

PHP

<?php

function flattenArray($arr) {
    $res = [];
    $stack = [$arr];

    while ($stack) {
        $current = array_shift($stack);

        foreach ($current as $value) {
            if (is_array($value)) {
                array_unshift($stack, $value);
            } else {
                $res[] = $value;
            }
        }
    }

    return $res;
}

// Driver code
$arr = [1, [2, 3, [4, 5]], 6];
$flatArr = flattenArray($arr);

print_r($flatArr);

?>

Output

Array
(
    [0] => 1
    [1] => 6
    [2] => 2
    [3] => 3
    [4] => 4
    [5] => 5
)

Approach 3: Using RecursiveIteratorIterator Class

PHP provides the RecursiveIteratorIterator class, this is used to flatten multidimensional arrays.

Example:

PHP

<?php

function flattenArray($arr) {
    $iterator = new RecursiveIteratorIterator(
        new RecursiveArrayIterator($arr)
    );

    return iterator_to_array($iterator, false);
}

// Driver code
$arr = [1, [2, 3, [4, 5]], 6];
$flatArr = flattenArray($arr);

print_r($flatArr);

?>

Output

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 6
)