<form action="subdir.php" method="post">
<input type="text" name="subdir">
<input type="submit" value="OK">
Then, in subdir.php, I would have this:
// Put user requested subdir into variable. Probably needs cleaning, in case of 'dodgey' url's containing things like .. or /'s
$subdir = $_POST['subdir'];
That will redirect the user to the subdir they entered - whether they pressed enter or clicked the button. Using PHP allows for expansion and a more secure environment to navigate folders in my opinion.