一个php数组转字符串的问题

数组降维么/？不怎么懂……

$array = array(
    array(
        'device_token' => 'Al9_G0i6ftf7fvkKsGM9o6jN5iyoqt8zTjcsh_kw6HUu'
    ),
    array(
        'device_token' => 'AmfUS3qeXKrJt1K1ZTICiD-ED6a_YgM3GdBRp6gR4RgV'
    )
);
$result = [];
array_walk_recursive($array, function ($value, $key) use (&$result) {
    $result[$key][] = $value;
});
foreach ($result as $key => $value) {
    $result[$key] = implode($value, ',');
}
$result = json_encode($result);
// 如果你确定你不是想要json格式的话就保留下面
$result = substr($result, 1, -1);