First, make sure the form tag has the following
<form method="post" enctype="multipart/form-data">
Second, access the file like this:
<input type="file" name="file">
move_uploaded_file($_REQUEST['file']['tmp_name'],'/tmp/blah');
This moves the file to a new location. Also access:
$REQUEST['file']['name']
$REQUEST['file']['size']
$_REQUEST['file']['type']