php - Can't get access to database in user access class -

i'm new in oop , access class db in user_access class. can't, error , can't figure out why errors that:

warning: mysqli_query() expects parameter 1 mysqli, object given in c:\xampp\htdocs\brothers.traning\profiadmin\models\login_class.php on line 20  notice: trying property of non-object in c:\xampp\htdocs\brothers.traning\profiadmin\models\login_class.php on line 21 not 

this database class:

    class db {     public function db()     {         $db = new mysqli(db_server, db_username, db_password, db_database);           if (mysqli_connect_errno()) {              echo "error: not connect database.";              exit;          }      }  } 

and im user_access class:

class user_access{      public function __construct($db){         $this->db = $db;      }     public function check_login($login,$password){         $password = sha1($password);         $sql = "select * admins username = '$login' , password = '$password'";         $result = mysqli_query($this->db,$sql);         if($result->num_rows > 1){             echo 'ok';         }else{             echo "not";         }     }  } 

now when use:

  $db = new db();  $userdb = new user_access($db);      $userdb->check_login('artur','ol'); 

i error... can me, , tell me wrong code..?

edit3: change

$result = mysqli_query($this->db,$sql); 

to

$result = $this->db->query($sql); 

look manual: http://php.net/manual/en/mysqli.query.php

you working in object-oriented style methods of making queries little bit different. in first line you've used procedural-style , wrong.

edit2: must return connection object!

class db { public function db() {     $db = new mysqli(db_server, db_username, db_password, db_database);       if ($db->connect_errno()) {          echo "error: not connect database.";          exit;      }     else { return $db; }  }  } 

and change way call it:

$userdb = new user_access($db->db()); 

edit:

class user_access{  public $db;   public function __construct($db){     $this->db = $db;  } public function check_login($login,$password){     $password = sha1($password);     $sql = "select * admins username = '$login' , password = '$password'";     $result = mysqli_query($this->db,$sql);     if($result->num_rows > 1){         echo 'ok';     }else{         echo "not";     } }  } 

question: in user_access class:

public function __construct($db){     $this->db = $db;  } 

where $db variable want access $this->db.