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
 ****************