Add an ext/hash compatibility layer (just hash_pbkdf2(), for now)
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 9907e97..5d5c5df 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -508,7 +508,7 @@
- Changed method ``clean_string()`` to utilize ``mb_convert_encoding()`` if it is available but ``iconv()`` is not.
- Renamed method ``_is_ascii()`` to ``is_ascii()`` and made it public.
- - Added `compatibility layers <general/compatibility_functions>` for PHP's `mbstring <http://php.net/mbstring>`_ (limited support) and `password <http://php.net/password>`_ extensions.
+ - Added `compatibility layers <general/compatibility_functions>` for PHP's `mbstring <http://php.net/mbstring>`_ (limited support), `hash <http://php.net/hash>`_ and `password <http://php.net/password>`_ extensions.
- Removed ``CI_CORE`` boolean constant from *CodeIgniter.php* (no longer Reactor and Core versions).
- Log Library will now try to create the **log_path** directory if it doesn't exist.
- Added support for HTTP-Only cookies with new config option *cookie_httponly* (default FALSE).
diff --git a/user_guide_src/source/general/compatibility_functions.rst b/user_guide_src/source/general/compatibility_functions.rst
index e025d2a..3495101 100644
--- a/user_guide_src/source/general/compatibility_functions.rst
+++ b/user_guide_src/source/general/compatibility_functions.rst
@@ -93,6 +93,35 @@
For more information, please refer to the `PHP manual for
password_verify() <http://php.net/password_verify>`_.
+*********************
+Hash (Message Digest)
+*********************
+
+This compatibility layer contains only a single function at
+this time - ``hash_pbkdf2()``, which otherwise requires PHP 5.5.
+
+Dependancies
+============
+
+- None
+
+Function reference
+==================
+
+.. function:: hash_pbkdf2($algo, $password, $salt, $iterations[, $length = 0[, $raw_output = FALSE]])
+
+ :param string $algo: Hashing algorithm
+ :param string $password: Password
+ :param string $salt: Hash salt
+ :param int $iterations: Number of iterations to perform during derivation
+ :param int $length: Output string length
+ :param bool $raw_output: Whether to return raw binary data
+ :returns: Password-derived key or FALSE on failure
+ :rtype: string
+
+ For more information, please refer to the `PHP manual for
+ hash_pbkdf2() <http://php.net/hash_pbkdf2>`_.
+
****************
Multibyte String
****************