PDO Version

init.php
if (logged_in () === true) {
    $user_data = user_data($db, $session_user_id, 'email');
}

functions.php
function user_data(PDO $db, $session_user_id) {
    $data = array();
    $user_id = (int)$user_id;
    $func_num_args = func_num_args();
    $func_get_args = func_get_args();
    if ($func_num_args > 1) {
        unset($func_get_args[0]);
        $fields = '`' . implode('`, `', $func_get_args) . '`';
        $query = $db->prepare("SELECT $fields FROM `admin` WHERE `id` = :user_id");
        $query->bindParam(":user_id", $user_id);
        $query->execute();
        $data = $query->fetch(PDO::FETCH_ASSOC);
        return $data;
    }
}

Once again
echo $user_data['email'];

result: nothing