Wie kann ich eine Datenbankverbindung in einer Benutzerklasse öffnen, in der ich Datenbankoperationen durchführen kann? und warum müssen eingebaute erstellte Funktionen in der DBConnection-Klasse definiert werden?

ich habe erschaffen

db.phpuser.phpresult.php

in db.php

class DBConnection  
{       
  protected $mysqli;
  private  $db_host="127.0.0.1";
  private  $db_name="test";
  private  $db_username="root";
  private  $db_password="";

  public function __construct()
    {
        $this->mysqli=new mysqli($this->db_host,$this->db_username,
                $this->  db_password,$this->db_name) or die($this->mysqli->error);

         return $this->mysqli;
    }

public function query($query)  // why i need to creat this function
     {
    return $this->mysqli->query($query);
     }

public function real_escape_string($str)
 {
    return $this->mysqli->real_escape_string();
 }


   function __destruct(){
     //Close the Connection
     $this->mysqli->close();
    }
}
?>

in User.php

<?php
  require "db.php"; 

  class User {  

  public function __construct($conn)
{
    $this->mysqli=$conn;

}

   public function addUser($name,$username,$email,$pwd)
   {
     $query="  ";

     $res=$this->mysqli->query($query);

  //pls chek the query function in DBConnection,what is the need to define query               >       function in DBConnection ?

    return $res;
    }
   }    
?>

in result.php

    <?php

      require "user.php";

      $conn=new DBConnection();

      if(isset($_POST['submit']))
  {

 $name = $conn->real_escape_string(trim(strip_tags($_POST['name'])));
   $username =  $conn->real_escape_string(trim(strip_tags($_POST['username'])));
   $email =     $conn->real_escape_string(trim(strip_tags($_POST['email'])));
   $password    = $conn->real_escape_string(trim(strip_tags($_POST['pass'])));

//echo $name."".$username."".$email."".$password;
  $uObj=new User($conn);
  $uObj->addUser($name,$username,$email,$password);


echo " hi your name is <I>".$name."</I> and your email ID is <I>".$email."</I>";
        }


?>