$data = [
    1, 2, 4, 6,
    [
        2, 5, 6, 7,
        [
            9, 12, 55, 66, 77
        ]
    ],
];

function eatArr(array $arr)
{
    foreach ($arr as $item) {
        if (is_array($item)) {
            eatArr($item);
        } else {
            echo $item . ' ';
        }
    }
}

eatArr($data);