PHP header not working

[oscarinfo]

I have this code:

    <?php
    // *** Validate request to login to this site.
    if (!isset($_SESSION)) {
      session_start();
    }
 
    $loginFormAction = $_SERVER['PHP_SELF'];
    if (isset($_GET['accesscheck'])) {
      $_SESSION['PrevUrl'] = "index.html";
    }
    // Declaring variables
    if (isset($_POST['correo'])) {
      $loginUsername=$_POST['correo'];
      $password=$_POST['clave'];
      $MM_fldUserAuthorization = "";
      $MM_redirectLoginSuccess = "index.html";
      $MM_redirectLoginFailed = "entrar.php";
      $MM_redirecttoReferrer = false;
      mysql_select_db($database_slotcitymx, $slotcitymx);
      //Checking if teh user is in the database
      $LoginRS__query=sprintf("SELECT sj_correo, sj_clave FROM sc_jugador WHERE sj_correo=%s AND sj_clave=%s",
        GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text")); 
 
      $LoginRS = mysql_query($LoginRS__query, $slotcitymx) or die(mysql_error());
      $loginFoundUser = mysql_num_rows($LoginRS);
      if ($loginFoundUser) {
         $loginStrGroup = "";
 
        //declare two session variables and assign them
        $_SESSION['MM_Username'] = $loginUsername;
        $_SESSION['MM_UserGroup'] = $loginStrGroup;          
 
        if (isset($_SESSION['PrevUrl']) && false) {
          $MM_redirectLoginSuccess = $_SESSION['PrevUrl'];    
        }
        header("Location: " . $MM_redirectLoginSuccess );
      }
      else {
        header("Location: ". $MM_redirectLoginFailed );
      }
    }
    ?>

When I run it from dreamweaver it works fine but if I call this page from my main page it doesn't do anything. Any idea why? Thank you

[Dormilich]

I’d say line #3. $_SESSION is available after you call session_start().

and if you don’t submit the form there shouldn’t happen anything either.