PHP array_flatten() function. Convert a multi-dimensional array into a single-dimensional array.

array_flatten.php

<?php 

/**
 * Convert a multi-dimensional array into a single-dimensional array.
 * @author Sean Cannon, LitmusBox.com | [email protected]
 * @param  array $array The multi-dimensional array.
 * @return array
 */
function array_flatten($array) { 
  if (!is_array($array)) { 
    return false; 
  } 
  $result = array(); 
  foreach ($array as $key => $value) { 
    if (is_array($value)) { 
      $result = array_merge($result, array_flatten($value)); 
    } else { 
      $result = array_merge($result, array($key => $value));
    } 
  } 
  return $result; 
}

For a simple 2D array:

$multiDimArray = [[1, 2], [3, 4], [5, 6]];
$flatArray = array_reduce($multiDimArray, 'array_merge', []);

Using variadic: $result = array_merge(...$array);

As was previously stated, this doesn't work for associative arrays, but if you really don't care about the keys, you can try this:

array_merge(...array_values($array))

Tested on PHP 8.2.8

for my task i was needed function for flatten multidimensional array but i also needed keep keys like values, so i add little changes for initial answer and get what i needed

function array_flatten($array) { 
  if (!is_array($array)) { 
    return false; 
  } 
  $result = array(); 
  foreach ($array as $key => $value) { 
    if (is_array($value)) { 
        //just add [$key]
      $result = array_merge($result, [$key], array_flatten($value)); 
    } else { 
      $result = array_merge($result, array($key => $value));
    } 
  } 
  return $result; 
}

$etalonMap = [
    'OneItem' => [
         'item_1' => [
            'item_1_1',
            'item_1_2',
            'item_1_3'
        ],
        'item_2',
        'item_3',
        'item_4',
        'item_5',
    ],
    'TwoItem'=> [
        'TwoItem_1',
    ]
];
print_r(array_flatten($etalonMap));
/**
 * Array
(
    [0] => OneItem
    [1] => item_1
    [2] => item_1_1
    [3] => item_1_2
    [4] => item_1_3
    [5] => item_2
    [6] => item_3
    [7] => item_4
    [8] => item_5
    [9] => TwoItem
    [10] => TwoItem_1
)
**/


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