If you can convert a two-dimensional array into a one-dimensional array gracefully and quickly, or convert a multi-dimensional array into a one-dimensional array

How is it the simplest , Quickly expand and tile multi-dimensional arrays into one-dimensional arrays ? The following is the summary of the blogger for reference : Divided into tiles, the depth of expansion is 2 And all tiled :

Let's assume that there is an array to expand :

$arr = [
    'a' => [
        'a1' => 'nice100.net',
        'a2' => 'good',
    ],
    'b' => [
        'b1' => [
            'b11' => 'nice',
            'b12' => 'like',
        ]
    ]
];

• The depth of tiling is 2:

// Method 1 
$result = call_user_func_array('array_merge', $arr);
// Method 2 ( Will be faster )
$result = array_merge(...array_values($arr));
// Results output 
Array
(
    [a1] => nice100.net
    [a2] => good
    [b1] => Array
        (
            [b11] => nice
            [b12] => like
        )

)

• Tile all

// If you want to tile them all , So for multidimensional arrays , array_merge Function can't satisfy , Recursion needs to be handled 
$re = [];
array_walk_recursive($arr, function ($v, $k) use (&$re) {
    $re[$k] = $v;
}, $re);
// Input result , Note that if there is the same key, Then the latter will cover the former 
Array
(
    [a1] => nice100.net
    [a2] => good
    [b11] => nice
    [b12] => like
)

From my blog (https://blog.nice100.net/default/45.html)