Call to a member function prepare() on null [duplicate]

Answer 1

In your public function ConnessioneDB() in the Database classs you need to return the connection

class Database
{
    private $host = "localhost";
    private $nome_db = "GED";
    private $username = "root";
    private $password = "root";
    public $conn;

    public function ConnessioneDB()
    {

        $this->conn = null;
        try
        {
            $this->conn = new PDO("mysql:host=".$this->host.";dbname=".$this->nome_db, $this->username,$this->password);
            $this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            $this->conn->exec("SET NAMES utf8");

            return $this->conn;


        }
        catch(PDOException $e)
        {
          echo  "<div id='error-top'>Si &egrave; verificato un errore di connessione.</div>";
        }
    }
}

If you do not return the connection the $conn property in your User class will be null. Thus you get this error.