diff -u b/includes/cache.inc b/includes/cache.inc
--- b/includes/cache.inc
+++ b/includes/cache.inc
@@ -102,7 +102,7 @@
  *   1MB in size to be stored by default. When caching large arrays or similar,
  *   take care to ensure $data does not exceed this size.
  * @param $bin
- *   The cache bin to store the data in. Valid core values are:
+ *   (optional) The cache bin to store the data in. Valid core values are:
  *   - cache: (default) Generic cache storage bin (used for theme registry,
  *     locale date, list of simpletest tests, etc.).
  *   - cache_block: Stores the content of various blocks.
@@ -121,7 +121,7 @@
  *     the administrator panel.
  *   - cache_path: Stores the system paths that have an alias.
  * @param $expire
- *   One of the following values:
+ *   (optional) One of the following values:
  *   - CACHE_PERMANENT: Indicates that the item should never be removed unless
  *     explicitly told to using cache_clear_all() with a cache ID.
  *   - CACHE_TEMPORARY: Indicates that the item should be removed at the next
@@ -252,17 +252,16 @@
   /**
    * Stores data in the persistent cache.
    *
-   * @param $cid
+   * @param int $cid
    *   The cache ID of the data to store.
    * @param $data
    *   The data to store in the cache. Complex data types will be automatically
-   *   serialized before insertion.
-   *   Strings will be stored as plain text and not serialized.
-   *   Some storage engines only allow objects up to a maximum of 1MB in size to
-   *   be stored by default. When caching large arrays or similar, take care to
-   *   ensure $data does not exceed this size.
+   *   serialized before insertion. Strings will be stored as plain text and not
+   *   serialized. Some storage engines only allow objects up to a maximum of
+   *   1MB in size to be stored by default. When caching large arrays or
+   *   similar, take care to ensure $data does not exceed this size.
    * @param $expire
-   *   One of the following values:
+   *   (optional) One of the following values:
    *   - CACHE_PERMANENT: Indicates that the item should never be removed unless
    *     explicitly told to using cache_clear_all() with a cache ID.
    *   - CACHE_TEMPORARY: Indicates that the item should be removed at the next