// model
public function get(string $field, string $value)
{
    $database = Database::getInstant();
    $sql = "SELECT * FROM :table WHERE :field = :value;";
    $result = $database->query($sql, [
        ':table' => $this->table,
        ':field' => $field,
        ':value' => $value,
    ], static::class);
    return $result;
}

// database
public function query(string $sql, array $params = [], string $className = 'stdClass')
{
    $sth = $this->pdo->prepare($sql);
    $result = $sth->execute($params);
    if ($result === false) return null;
    return $sth->fetchAll(PDO::FETCH_CLASS, $className);
}