imap_list returns a simple array of strings that look just like the $ref parameter one needs to specify:

<?php

  $srv = '{imap.example.com}';

  $conn = imap_open($srv, 'me', 'mypass');

  $boxes = imap_list($conn, $srv, '*');

  print_r($boxes);
  imap_close($conn);
?>

Output:

<pre>
Array
(
    [0] => {imap.example.com}Calendar
    [1] => {imap.example.com}Contacts
    [2] => {imap.example.com}Deleted Items
    [3] => {imap.example.com}Drafts
    [4] => {imap.example.com}Journal
    [5] => {imap.example.com}Junk E-mail
    [6] => {imap.example.com}Notes
    [7] => {imap.example.com}Outbox
    [8] => {imap.example.com}RSS Feeds
    [9] => {imap.example.com}Sent Items  
    [10] => {imap.example.com}Tasks
)
</pre>

These directly fill in the $ref argument for every function that takes one.