OnChange for selectbox

[acctman]

I want to set a session id [folder] automatically when a user select one of the option an onChange event should be sent which sets a session. This should happen without having to use a submit box

<form action="" method="post" enctype="multipart/form-data">
Step 1: <strong>Save Photo to: </strong><select name="folder"">
<option value="" selected="selected">Select</option>
<option value="1">Public</option>
<option value="2">Mobile</option>
<option value="3">Private</option>
</select> Folder
</form>