Variabel fra en funktion til en anden (Query, class)

Kode 

class db {
	private $query, $db;
	private $row;
	
	function __construct() {
	}
	public function selectAll($db) {
		global $conn;
		$query = "SELECT * FROM $db";
		$result = $conn->query($query);
		$row = $result->fetch_object();
		$this->row=$row;
		return $row;
	}
	public function getDataGrid() {
		$res = "<table border=1>
		<tr><td>ID</td><td>".$this->row->id."</td></tr>
		<tr><td>Overskrift</td><td>".$this->row->overskrift."</td></tr>
		<tr><td>Tekst</td><td>".$this->row->tekst."</td></tr>
		<tr><td>Dator</td><td>".$this->row->dato."</td></tr>
		</table>";
		return $res;
	}
}

PHP kode 

 <?php 
 class db {
  
      private $query, $db, $conn
  
      function __construct() {
          /* opsæt din database connection her */
          $this->conn = "xxxx";
      }
  
      public function selectAll($db) {
          $query = "SELECT * FROM $db";
          $result = $this->conn->query($query);
          return $result->fetch_object();
      }
  
      public function getDataGrid() {
          $row = $this->selectAll($this->db);
          $res = "<table border=1>
                  <tr><td>ID</td><td>".$row->id."</td></tr>
                  <tr><td>Overskrift</td><td>".$row->overskrift."</td></tr>
                  <tr><td>Tekst</td><td>".$row->tekst."</td></tr>
                  <tr><td>Dator</td><td>".$row->dato."</td></tr>
                  </table>";
          return $res;
      }
  
  }