$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
    ....
  }
}

require_once 'api/core.php';
$database = new Database($pdo);
$user = new User($_SERVER['REMOTE_ADDR'], $_SESSION['info']['id']);