Fatal error: Call to a member function query() on a non-object in

<?
class Db{
  private $connection;
  private static $db;

  public static function getInstance($option = null){
    if (self::$db == null){
      self::$db = new Db($option);
    }

    return self::$db;
  }

  public function __costruct($option = null){
    if ($option != null){
      $host = $option['host'];
      $user = $option['user'];
      $pass = $option['pass'];
      $name = $option['name'];
    }else{
      global $config;
      $host = $config['db']['host'];
      $user = $config['db']['user'];
      $pass = $config['db']['pass'];
      $name = $config['db']['name'];
    }

    $this->connection = new mysqli($host, $user, $pass, $name);
    if($this->connection->connect_error){
      echo "connection failed: " . $this->connection->connect_error;
      exit;
    }
    $this->connection->mysql_query("SET NAMES 'utf8'");
  }



  public function first($sql){
    $records = $this->query($sql);
    if ($records == null){
      return null;
    }
    return $records[0];
  }

  public function  query($sql){
    $result = $this->connection->query($sql);
    $records = array();

    if ($result->num_rows == 0) {
      return null;
    }
    while($row = $result->fetch_assoc()){
      $records[] = $row;
    }
    return $records;
  }


  public function connection(){
    return $this->connection;
  }

  public function close(){
    $this->connection->close();
  }
}
?>



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