Expires data from the cache.
If called without arguments, expirable entries will be cleared from the cache_page and cache_block bins.
$cid: If set, the cache ID or an array of cache IDs. Otherwise, all cache entries that can expire are deleted. The $wildcard argument will be ignored if set to NULL.
$wildcard: If TRUE, the $cid argument must contain a string value and cache IDs starting with $cid are deleted in addition to the exact cache ID specified by $cid. If $wildcard is TRUE and $cid is '*', the entire cache is emptied.
function clear($cid = NULL, $wildcard = FALSE);