Сергей Яковлев

$arr = [
    ["name" => "Austin"],
    ["name" => "Nashville"],
    ["name" => "Foldo"],
    ["name" => "Femosss"]
];

function arrayFilterFirstChar(array $arr, string $chr): array
{
    return array_values(array_filter($arr, fn($val) => $val['name'][0] == $chr));
}

var_dump(arrayFilterFirstChar($arr, 'F'));

$json = '{
    "result": {
        "Random ItemName": [
            {
                "user": "user1",
                "time_upd": "2020-12-09 12:25:03"
            }, {
                "user": "user2",
                "time_upd": "2020-12-09 12:25:03"
            }, {
                "user": "user3",
                "time_upd": "2020-12-09 12:25:03"
            }
        ]
    }
}';
$data = json_decode($json, true);
$i = 0; // меняется в цикле
$user_name = $data['result']['Random ItemName'][$i]['user'];
$time_upd = $data['result']['Random ItemName'][$i]['time_upd'];

$user_name = $data['result'][array_key_first($data['result'])][$i]['user'];
$time_upd = $data['result'][array_key_first($data['result'])][$i]['time_upd'];

foreach ($data['result'] as $arrayKey => $arrayValue) {
    foreach ($arrayValue as $arraySubvalue) {
        $user_name = $arraySubvalue['user'];
        $time_upd = $arraySubvalue['time_upd'];
        // …
    }
}

http_response_code(404);
exit;

header('Location: …', true, 301);
exit;

header('Location: …', true, 307);
exit;