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/**

 * Act on block cache ID (cid) parts before the cid is generated.

 *

 * This hook allows you to add, remove or modify the custom keys used to

 * generate a block cache ID (by default, these keys are set to the block

 * module and delta). These keys will be combined with the standard ones

 * provided by drupal_render_cid_parts() to generate the final block cache ID.

 *

 * To change the cache granularity used by drupal_render_cid_parts(), this hook

 * cannot be used; instead, set the 'cache' key in the block's definition in

 * hook_block_info().

 *

 * @params $cid_parts

 *   An array of elements used to build the cid.

 * @param $block

 *   The block object being acted on.

 *

 * @see _block_get_cache_id()

 */

function hook_block_cid_parts_alter(&$cid_parts, $block) {

  global $user;

  // This example shows how to cache a block based on the user's timezone.

  $cid_parts[] = $user->timezone;

}