How to get values of selected options in Form api?
Since the form submission handler has access to the $form variable, you just use the following code.

$foldername = $form['select_list']['#options'][$form_state['values']['select_list']];


To make it clear with an example: $form['select_list']['#options'] is an array like array( 1 =&gt; 'first folder', 2 =&gt; 'second folder'), and $form_state['values']['select_list'] is a value between 1 and 2.