<?php 
session_start ();//facciamo partire la sessione
//controlliamo i dati inviati    
 if ((!isset($_POST['utente'])) && (!isset($_POST['password']))) 
 { 
// ecco il form
  echo "<form method=\"post\" action=\"".$_SERVER['PHP_SELF']."\"> 
  User: <input type=\"text\" name=\"username\"><br> 
  Password: <input type=\"password\" name=\"password\"><br> 
  <input type=\"submit\" value=\"Invia\"></form>"; 
  }else{ 
// query in tabella
  $sql = mysql_query("SELECT * FROM tabella WHERE username='".$POST['username']."' AND '".$_POST['pasword']."'");
  if (mysql_num_rows($sql)>0)
  {
  $_SESSION['username']=$_POST['username']; 
  $_SESSION['password']=$_POST['password']; 
  header("Location: pagina_riservata.php");
  }
 else {echo "Login errato";} 
  }
?>