Flatten PHP array

result.txt

C:\>php flatten.php
array(4) {
  [0]=>
  int(11)
  [1]=>
  int(12)
  [2]=>
  int(13)
  [3]=>
  int(14)
}
array(3) {
  [0]=>
  int(21)
  [1]=>
  int(23)
  [2]=>
  int(24)
}
array(4) {
  ["a"]=>
  int(31)
  ["b"]=>
  int(32)
  ["c"]=>
  int(33)
  ["d"]=>
  int(34)
}
array(4) {
  ["a"]=>
  int(41)
  ["b"]=>
  int(42)
  ["c"]=>
  int(43)
  ["d"]=>
  int(44)
}

test.php

<?php

function array_flatten(array $array = null) 
{
    $result = [];
    if ($array) {
        foreach ($array as $key => $value) {
            $result += is_array($value) 
                ? array_flatten($value) 
                : [$key => $value];
        }
    }
    return $result;
}

var_dump(array_flatten(
    [11, 12, 13, 14]
));

var_dump(array_flatten(
    [
        [21, 
            [22, 23], 
        24]
    ]
));

var_dump(array_flatten(
    [
        'a' => 31, 
        ['b' => 32, 'c' => 33], 
        'd' => 34
    ]
));

var_dump(array_flatten(
    [
        'a' => 41, 
        [
            'b' => 42, 
            ['c' => 43], 
            'd' => 44
        ]
    ]
));