How to Flatten Multidimentional Array in PHP ?

Last Updated : 11 Jul, 2024

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:

Table of Content

Using Recursive Method
Iterative Method using Stack
Using RecursiveIteratorIterator Class
Using array_walk_recursive()
Using array_reduce() and array_merge()

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
)

Approach 4: Using array_walk_recursive()

PHP's array_walk_recursive() function provides a straightforward way to flatten a multidimensional array. This function applies a user-defined callback function to each element of the array recursively.

Example: The array_walk_recursive() approach provides a clean and efficient way to flatten a multidimensional array in PHP.

PHP

<?php
function flattenArray($array) {
    $flatArray = [];
    
    array_walk_recursive($array, function($value) use (&$flatArray) {
        $flatArray[] = $value;
    });
    
    return $flatArray;
}

// Example usage:
$arr1 = [[1, 2], [3, 4, 5], 6];
$arr2 = [[1, 4, 5], [6, 7, 8]];

print_r(flattenArray($arr1));
// Output: [1, 2, 3, 4, 5, 6]

print_r(flattenArray($arr2));
// Output: [1, 4, 5, 6, 7, 8]
?>

Output

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
    [4] => 5
    [5] => 6
)
Array
(
    [0] => 1
    [1] => 4
    [2] => 5
    [3] => 6
    [4] => 7
    [5] => 8
)

Approach 5: Using array_reduce() and array_merge()

This approach combines array_reduce() and array_merge() functions to flatten the multidimensional array. array_reduce() iterates over each sub-array and merges them into a single array.

Example: