/**
 * Denotes that a block is not enabled in any region and should not be shown.
 */
define('BLOCK_REGION_NONE', -1);

/**
 * The base URL of the drupal installation.
 *
 * @see conf_init()
 */
global $base_url;

Constants, global variables, and member variables of classes should have documentation blocks. As usual, the documentation block should start with a one-line description, and any additional needed documentation should be separated from the summary line by a blank line (though usually only one line is needed).