Add an ext/hash compatibility layer (just hash_pbkdf2(), for now)
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
****************