Call to a member function query() on string OOP PHP -

i'm new oop php im trying learn doing few basic things, error im getting in title query keeps returning error.

<?php     //connection starts here  class connection{    	private $host = 'localhost';  	private $username = 'root';  	private $password = '';  	private $database = 'test';  	public $conn;    	public function __construct(){    		return $this->conn = mysqli_connect($this->host, $this->username, $this->password, $this->database) or die("mysql error");    	}    }    //users starts here  class users{    	public $db;    	public function __construct($db){    		$this->db = $db;    	}    	public function getallusernames($db){    		$query = "select * users";            //error on line  		$all_users = $this->db->query("$query");    		while ($row = $all_users->fetch_assoc()) {  			return $row['username'];  		}    	}  }

i have no idea im going wrong can me better understanding of ive done wrong, know have clean lot of previous code said im learning , that's story want know error means , what's making it..?

you're not far of actually. altered code bit , instantiated classes:

<?php   //connection starts here class connection {      private $host = 'localhost';     private $username = 'root';     private $password = '';     private $database = 'test';     private $conn;      public function __construct(){          $this->conn = mysqli_connect($this->host, $this->username, $this->password, $this->database) or die("mysql error");      }      public function getconn(){         return $this->conn;     }  }  //users starts here class users {      private $db;      public function __construct($db){          $this->db = $db;      }      public function getallusernames(){          $query = "select * users";          //error on line         $all_users = $this->db->query("$query");         return $all_users;         } }  $connection = new connection(); $users = new users($connection->getconn());  $allusers = $users->getallusernames(); while ($row = $allusers->fetch_assoc()) {         echo $row['username']; }