Inside the block admini interface, one can enter PHP code snippets in “Page visibility settings” section of the block configuration page.
While a page being generated, Drupal will run the PHP snippet to determine whether a block will be displayed.

Examples of some of the most common snippets follow; each snippet should return TRUE or FALSE to indicate whether the block should be visible for that particular request.

Displaying a Block to Logged-In Users Only
Only return TRUE when $user->id is not 0.

<?php
   global $user;
   return (bool) $user->uid;
?>

Displaying a Block to Anonymous Users Only
Only return TRUE when $user->uid is 0.

<?php
   global $user;
   return !(bool) $user->uid;
?>