مشکل در اتصال به دیتابیس

سلام برای اتصال به دیتا بیس از روش استاد استفاده کردم .اما با این error مواجه شدم :

Fatal error: Call to a member function query() on a non-object in C:\dev\workspace\web\note\Db.php on line 38

class Db {

  private $connection;

  public function __construction($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 faild :".$this->connection->connect_error;
      exit;
    }

    $this->connection->query("SET NAMES 'utf8'");
  }

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

 public function query($sql){
 line 38 = $result=$this->connection->query($sql);
   if($result->num_rows==0){
     return null;
   }
   while($row=$result->fetch_assoc()){
     $records[]=$row;
   }
   return $records;
 }