<?php
global $user;
$allowed = array('authenticated user');
$valid=FALSE;
foreach($user->roles as $role){
  if(in_array($role, $allowed)) {
    $valid=TRUE;
  }
}
return $valid;

<?php
global $user;
$notallowed = array('anonymous user');
$valid=TRUE;
foreach($user->roles as $role){
  if(in_array($role, $notallowed)) {
    $valid=FALSE;
  }
}
return $valid;