PHP Arrays, trying to fix double array

I am new to arrays but I’m trying to achieve the following:

{
    "Basic": "91919191",
    "Info": [
            {
                "ID": 1,
                "Extra": 3
            },
            {
                "ID": 2,
                "Extra": 6
            }, etc etc..
    ]
}

But instead I get double array like this:

{
    "Basic": "91919191",
    "Info": [
        [
            {
                "ID": 1,
                "Extra": 3
            },
            {
                "ID": 2,
                "Extra": 6
            }, etc etc..
        ]
    ]
}

Here is the code I am currently using

$first = array();
$first['Basic'] = "91919191";

$x = 1;
while ($x <= 3) {
    $y = $x * 3;
    $second[] = array('ID' => $x, 'Extra' => $y);
    $x++;
}
$first['Info'] = array($second);

echo json_encode($first, JSON_PRETTY_PRINT);

I’ve tried using array_merge but the results are not as expected. Any ideas/suggestions?