Check if the IMAP stream is still active
(PHP 4, PHP 5)
bool imap_ping ( resourceimap_stream )
imap_ping() pings the stream to see if it's still
active. It may discover new mail; this is the preferred method for a
periodic "new mail check" as well as a "keep alive" for servers which
have inactivity timeout.
This is a great solution for a webmail class if you want to make sure that a user is still logged in. Example:
<?php
if(!(imap_ping($stream)) {
die("The user is no longer logged in.");
?>