$list_epoch = 1;

$list = array(
  'blah',
  'yadda',
  'etc'
);

if(($_SERVER['HTTP_REFERER'] == 'http://georgebushisyournewbicycle.com/') && !($_COOKIE['annoy_spacehobo'])) {
  # SpaceHobo stares at you disapprovingly.
  # Do not offer him cookies again, and instead serve a random entry.
  $phrasen = array_rand($list);
  $phrase = $list[$phrasen];
} else {
  # PHP session cookie junk
  session_name('annoy_spacehobo');
  session_start();

  # Just in case you need to invalidate everyone's list cache.
  # This also serves as array initialization for new visitors.
  if($_SESSION['list_epoch'] < $list_epoch) {
    $_SESSION['list_epoch'] = $list_epoch;
    $_SESSION['list_used'] = array();
  }

  # If the user has gone through all phrases, reset.
  if(count($_SESSION['list_used']) >= count($list)) {
    $_SESSION['list_used'] = array();
  }

  # Weed out entries the user has already seen.
  # NOTE: Figure out something more efficient if list ever gets
  # over a couple thousand.
  $list_candidates = array();
  foreach($list as $a) {
    if(!in_array($a, $_SESSION['list_used'])) {
      $list_candidates[] = $a;
    }
  }

  if(count($list_candidates) > 0) {
    $phrasen = array_rand($list_candidates);
    $phrase = $list_candidates[$phrasen];
    $_SESSION['list_used'][] = $phrase;
  } else {
    # The only reason $list_candidates should have zero entries is if
    # $list had duplicate entries.  Just in case, reset and serve up
    # something random.
    $_SESSION['list_used'] = array();
    $phrasen = array_rand($list);
    $phrase = $list[$phrasen];
  }
}