I have a login script that does this:

```
$_SESSION['username']=$username;

$_SESSION['password']=$password;

```

If the user logged in succesfully. 
And so I edited the signup page to do this:

```
<?php

function redirect() {
    header(' URL= index.php');
}

?>
<?php session_start(); ?>  

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en" id="jmtoday" class=" no_js">
<head>
 <link href='icon.jpg' rel='icon' type='image/jpg'/>
 <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
 <meta http-equiv="Content-language" content="en" />
 <LINK REL=StyleSheet HREF="Mainstyles.css" TYPE="text/css"></link>
 <Title>Sign up | JMToday</title>
</head>
<body>
<?php
  if(isset($_SESSION['username'])){
   redirect();
  }
?> 

```

But it doesn't redirect the user when I logged in with my account that I created. Why is that?