Poop连接到MySQL数据库的PHP oop继承

Question

I created two classes , USER and ADMIN , and admin extends user.

I am able to get which ever data i need from the database when using object of class USER but i cant get any data when working with the ADMIN object. the classes are as follows:

        class USER
        {
            private $conn;
            public function __construct()
            {
              $database = new Database();
              $db = $database->dbConnection();
              $this->conn = $db;
            }
            public function runQuery($sql)
            {
              $stmt = $this->conn->prepare($sql);
              return $stmt;
            }

        ...some functions to query the DB

        }

and

class ADMIN extends USER
{
private $conn;

public function __construct()
{
  $database = new Database();
  $db = $database->dbConnection();
  $this->conn = $db;
}
...some other functions to query the DB
}

at first i didn't include the constructor since i read that admin will inherit every not private property so this is my second try but in both cases i got this error:

Call to a member function prepare() on a non-object any idea what am i missing ? thx

UPDATE: i have this function for example in ADMIN class:

public function getAppeals($user_id){
  $stmt = $this->conn->prepare("SELECT * FROM appeals WHERE lecturer_id = :lecturer_id");
  $stmt->execute(array(':lecturer_id' => $user_id));
  $userRow = $stmt->fetchall(PDO::FETCH_ASSOC);
  return $userRow;
}

the row that generates the error is this:

$stmt = $this->conn->prepare("SELECT * FROM appeals WHERE lecturer_id = :lecturer_id");

Answer 1

This should explain your issue http://php.net/manual/en/language.oop5.visibility.php

A private property is only visible in the class in which it was defined. However, making it protected will allow it to be accessed from any class that extends USER so it will be visible to ADMIN assuming you run the parent constructor from the ADMIN class like this parent::__construct();

If you make these changes it should work

class USER
{
    //private $conn;
    protected $conn;

    public function __construct()
    {
      $database = new Database();
      $db = $database->dbConnection();
      $this->conn = $db;
    }
    public function runQuery($sql)
    {
      $stmt = $this->conn->prepare($sql);
      return $stmt;
    }

...some functions to query the DB

}

class ADMIN extends USER
{

    public function __construct()
    {
        parent::__construct();
    }

    public function getAppeals($user_id){
       $stmt = $this->conn->prepare("SELECT * FROM appeals WHERE lecturer_id = :lecturer_id");
       $stmt->execute(array(':lecturer_id' => $user_id));

       $userRow = $stmt->fetchall(PDO::FETCH_ASSOC);

       return $userRow;
    }
}