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; 
}

Nice @graceman9, but it doesn't work in PHP 7, as unpacking arrays with string keys was only implemented in PHP 8.

Fatal error: Uncaught Error: Cannot unpack array with string keys

Ref: https://wiki.php.net/rfc/array_unpacking_string_keys

I just changed ... with array_merge to make your code compatible with older versions of PHP.

function true_flatten(array $array, array $parents = [])
{
    $return = [];
    foreach ($array as $k => $value) {
        $p = empty($parents) ? [$k] : array_merge($parents, [$k]);
        if (is_array($value)) {
            $return = array_merge($return, true_flatten($value, $p));
        } else {
            $return[implode('_', $p)] = $value;
        }
    }

    return $return;
}

Output:

Array
(
    [a] => value1
    [b_c] => value1.1
    [b_d] => value1.2
    [e] => value2
    [f_j_k] => value2.1
    [f_j_m] => value2.2
    [f_j_n] => value2.3
    [o] => value3
    [p_some_very_deep_item_0] => first
    [p_some_very_deep_item_1] => second
    [q] => value5
)

Tested in PHP 7.4.9

@sayhicoelho Have you tried my approach above? It should work with associative and non-associative arrays, and be compatible with PHP 7+.

It also supports a depth parameter, so you can easily flatten only the top layer, the first N layers, or infinitely.

@brandonmcconnell Using my sample data it gives not what I expected.
By the way, how to use it with unlimited $depth? array_flatten($r, 999) doesn't look good for me
Despite of that your function does the job of flattening, no doubt :) maybe flatten is not the right word because of ambiguity?

@sayhicoelho thanks for 7.x support!

echo json_encode(array_flatten($r, 9));

{
    "a": "value1",
    "c": "value1.1",
    "d": "value1.2",
    "e": "value2",
    "k": "value2.1",
    "m": "value2.2",
    "n": "value2.3",
    "o": "value3",
    "0": "first",
    "1": "second",
    "q": "value5"
}

@graceman9 What data produces an unexpected result?

For infinite depth, just use -1

@brandonmcconnell Have you noticed the difference?

{
    "a": "value1",
    "b_c": "value1.1",
}

and

{
    "a": "value1",
    "c": "value1.1",
}

@graceman9 Oh you want the keys to be concatenated, so every key keeps the log of its original path. Yeah, you are correct that my version does not do that. I tried to follow the approach common in other languages. 🙂

For a simple 2D array:

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