How to avoid overwritting variable value

[xox]

Hello, I have a form that has a preview, before user can preview the form inputs he must login. If he is logedin, he can see preview and continue to editing form, but after user is finished with editing his values he is redirected to "preview" page for which he must be logged in. But the problem is that variables are "empty" after redirecting back to preview.

 

$_POST['email'] is received from login.php

$_SESSION['code'] is 5 digit code generated at start, it isn't important in this issue.

 

Here's the code:

 

preview.php

$id_user= 0;
$code = $_SESSION['code'];
    $email = $_POST['email'];
    $_SESSION['email_login'] = $email;

 

So now you can see that $email and $_SESSION['email_login'] are empty because when redirecting from "editing" to preview  $_POST['email'] can't be found...

 

[Psycho]

if(isset($_POST['email']) && $_POST['email']!='')
{
    $email = $_POST['email'];
    $_SESSION['email_login'] = $email;
}
elseif (isset($_SESSION['email_login']))
{
    $email = $_SESSION['email_login'];
}
else
{
    $email = false;
} 

[xox]

grazie