I have created a separate class for database and users.
Database.php
class Database{ private $db; public function __construct(){ /***
Your Database class does not extend PDO neither it does implement prepare method.
Database
prepare method
In order to access your PDO object, you must make it public and access like:
PDO
From User class:
User
$this->db->db->prepare();
The best way would be to extend the PDO class.