With this code everything works:
<?php
include("header.php");
// session_start();
?>
<body align='center'>
<div id='wrapper'>
<div id='login_form'>
<form action='login.php' method='post'>
<h1> Einloggen ins Forum.</h1>
<table>
<tr><td>Benutzername :</td> <td><input type='text' name='username'/></td></tr><br>
<tr><td>Passwort :</td> <td><input type='password' name='password'/></td></tr><br>
</table>
<input type='submit' name='login' value='Log in'/>
</form>
</div>
</div>
</body>
<?php
include("footer.php");
?>
but when the "session_start();" is not uncommented the browser can not find this page.. I found nothing on the internet so thats why I asked you