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 | seanc@litmusbox.com
 * @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; 
}

Working with associative arrays:

/**
    * flatten function
    *
    * @param array $multiDimArray
    * @return array
    */
function flatten(array $multiDimArray): array
{
    $localFlatten = [];

    foreach ($multiDimArray as $key => $value) {
        if (\is_array($value)) {
            foreach (flatten($value) as $subKey => $subValue) {
                $localFlatten[$subKey] = $subValue;
            }
            continue;
        }

        $localFlatten[$key] = $value;
    }

    return $localFlatten;
}

Using

flatten([['abc' => 123, 'a' => 1, 'abc'], [2 => 'zxc']]);
  // Result:
  [
    "abc" => 123,
    "a" => 1,
    0 => "abc",
    2 => "zxc",
  ]