<form method="post" action="login.php"
<input type="text" name="username"<br>
<input type="password" name="password"<br>
<input type="submit" value="login"
</form
<?php
header('Location:[url]http://[/url]'.$_POST['username'].':'.$_POST['password'].'@[url]www.mydomain.de/htacces_dir/[/url]);
?>