Call to a member function prepare() on null OOP - php

Question

Having trouble understanding classes and inheritance:
core.php:
$servername = "****";
$database = "****";
$username = "****";
$password = "****";
try {
$pdo = new PDO("mysql:host=$servername;dbname=$database", $username, $password);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch(PDOException $e) {
echo "Connection failed: " . $e->getMessage();
}
class Database {
protected $pdo;
public function __construct($pdo) {
$this->pdo = $pdo;
}
}
class User extends Database {
private $ip;
private $sessionId;
public function __construct($ip, $sessionId) {
$this->ip = $ip;
$this->sessionId = $sessionId;
}
public function getSessionInfo () {
$stmt = $this->pdo->prepare(".."); <-- error here
....
}
}
When calling:
require_once 'api/core.php';
$database = new Database($pdo);
$user = new User($_SERVER['REMOTE_ADDR'], $_SESSION['info']['id']);

Answer 1

In this contest $database, and $user variables are not related to each other:
require_once 'api/core.php';
$database = new Database($pdo);
$user = new User($_SERVER['REMOTE_ADDR'], $_SESSION['info']['id']);
Thus, calling prepare() on $user won't work.
You need a mechanism, at least like this , although not a good practice to assign Database to a User:
$user->setDatabase($database);
Instead create a static Database object, initiate it before User initiation, and call it statically within User object, or any other object, make it available for all objects.
A quick fix would look like this, where User doesn't extend Database, because it's wrong. User is not a Database.
$database = new Database();
$user = new User();
$user->setDatabase($database); //sets $db variable inside User
//User.php
namespace MyApp;
class User{
private Database $db;
public function setDatabase($db){
$this->db = $db;
}
public function doSomething(){
$this->db->getPdo()->prepare('..');
}
}
//Database.php
namespace MyApp;
class Database{
private $pdo; //returns PDO object
function __construct(){
//create pdo connection
$this->pdo = ..
}
function getPdo(){
return $this->pdo;
}
}
Database should be injected to objects or used by objects, you shouldn't be extending Database just to have it. If you want to do it properly, in an object-oriented way.
Remember PHP doesn't allow multiple inheritances by extend. Tomorrow, you might want to have a Person class that every User will extend, but since you did it wrong in the beginning, and wasting precious extend on Database, it won't be possible. And by not having a control of how many database instances you have created, you will run into issues. You need to know for sure that you have only a single connection object for one database, if of course the opposite is a must - which in your case I doubt.
Of course this will change if you have multiple database requirements, and more sophisticated app structure.

Answer 2

You are receiving this error because User Instance has pdo empty. try this code
$servername = "****";
$database = "****";
$username = "****";
$password = "****";
try {
$pdo = new PDO("mysql:host=$servername;dbname=$database", $username, $password);
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch(PDOException $e) {
echo "Connection failed: " . $e->getMessage();
}
class Database {
protected $pdo;
public function __construct($pdo) {
$this->pdo = $pdo;
}
}
class User extends Database {
private $ip;
private $sessionId;
public function __construct($pdo, $ip, $sessionId) {\
parent::__construct($pdo)
$this->ip = $ip;
$this->sessionId = $sessionId;
}
public function getSessionInfo () {
$stmt = $this->pdo->prepare("..");
....
}
}
then
require_once 'api/core.php';
$user = new User($pdo, $_SERVER['REMOTE_ADDR'], $_SESSION['info']['id']);
hope it helps.