Since imap_search returns an array, you can just use regular array functions:
$emails = imap_search($inbox,'UNSEEN SUBJECT "Task has succeeded"');
$last = array_pop($emails);
$overview = imap_fetch_overview($inbox,$last,0);
$message = imap_fetchbody($inbox,$last,2);
$output = '<div class="toggler '.($overview[0]->seen ? 'read' : 'unread').'">';
$output.= '<span class="subject">'.$overview[0]->subject.'</span> ';
$output.= '<span class="from">'.$overview[0]->from.'</span>';
$output.= '<span class="date">on '.$overview[0]->date.'</span>';
$output.= '</div>';
$output.= '<div class="body">'.$message.'</div>';
echo $output;