PHP二维数组（或任意维数组）转换成一维数组的方法

$arr = [
    'a' => [
        1,
        2,
    ],
    'b' => [
        '3',
        '4'=>[
            '5'
        ]
    ],
    'b'
];
$arr = [
    'a' => [
        1,
        2,