php object oriented class connect db [error to implement]

<?php

class Database {

    private $nomeTabella;
    private $conn;

    public function connettiAlDB(){
        $username = "prova"; $password = "gtgt";
        try{
                // connessione
                $conn = new PDO("mysql:host=localhost;dbname=my_tesinandroid", $username, $password);
                // abilita gestione errori tramite try ? catch (Exception)
                $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            } catch(PDOException $ex){
                die("Errore connessione: ".$ex->getMessage());
            }
    }

    public function selezionaTabella($nomeTabella){
        $this->$nomeTabella = $nomeTabella;
    }

    public function scriviSulDB($sql){

      //SCRIVERE SU DB

      try {
        $stmSql = $conn->prepare($sql);
        //    $stmSql ->bindParam(1, $nome);
        //    $stmSql ->bindParam(2, $telefono);
        $risultato = $stmSql ->execute();
        return $risultato;
      } catch (PDOException $ex) {
        echo("Query errata".$ex->getMessage());
      }

    }


  public function leggiDalDB($sql){

      //LEGGERE DA DB

      try {
        $stmSql = $conn->prepare($sql);
        $result = $stmSql ->execute();
        return $result;
      } catch (PDOException $ex) {
        die("Query errata".$ex->getMessage());
      }
  }

}

?>