Fatal error: Call to undefined method Database::prepare()

You create $dbh when you instantiate Database(), but instantiating the Database only returns an instance of your Database class, not your db connection. You should have a getDb to get your connection from database object:

$dbClass = new Database();
$dbh = $dbClass->getDb(); // here you get the connection
$users= new Users($dbh);  // here you give to Users() the $dbh, that isn't your 
                          // connection.. it's just Database class

Database construct only return an instance of your Database class, not your db connection

class Database{

 private $db;


public function __construct(){

    try {
     $this->db = new PDO("mysql:host=$hostname;dbname=kamadhenu_web", $username, $password);
    /*** echo a message saying we have connected ***/

   }
    catch(PDOException $e)
        {
            echo $e->getMessage();
       }    
 }

 public function getDb() {
       if ($this->db instanceof PDO) {
            return $this->db;
       }
 }


}