fixed

<?php
     session_start();
     function loginform() {
       echo " <form action='' method='post'>
              Usernane: <input type='text' name='username'/>
              Password: <input type='text' name='password'/>
                        <input type='submit' name='login' value='Login'/>
                        </form>";

     }
      function logoutform() {
       echo " <form action='' method='post'>
              <input type='submit' name='logout' value='Logout'/>
              </form>";
     }

      function login($username, $password) {
        $pass=md5($password);
        $conn=mysql_connect("localhost", "root", "")or die(mysql_error());
        mysql_select_db("registrazione", $conn) or die (mysql_err());
        $result=mysql_query("SELECT * FROM user WHERE username='$username' AND password='$pass' ") or die (mysql_err());
        $count= @mysql_num_row($result);

        if ($count==1){
            $_SESSION['login']=$username;
            echo "Login avvenuto con successo....";
        }
        else {
        echo "login errato. Controlla password e username...";
        }
     }
      function logout(){
        $session_destroy();
      }
     if (isset($_SESSION['login'])){
         echo "sei loggato, puoi uscire facendo click sul logout sotto<br>";
         logoutform();
     }else{
          echo "non sei loggato, fai i login...";
       loginform();
     }
     if ($_POST['login']){
     echo "login in corso.....";

     login($_POST['username'], $_POST['password']);
     }
     elseif ($_POST['logout']){
     echo "stai facendo il logout....";
     logout();
     }
?>