Description:
------------
When passing 0 as the length to hash_pbkdf2(), the behaviour is inconsistent 
between raw output and hex-encoded output. See the test script for a concrete 
example.

There are also no tests for the use case where length is defaulted in the current 
test suite (http://bit.ly/YcgMMu).

I'm attempting to write a compatibility library for this function for PHP versions 
earlier that 5.5, so the test suite for https://github.com/eloquent/pbkdf2-compat 
and its build history in Travis CI may also be useful, as it documents the current 
behaviour more extensively.

Test script:
---------------
<?php

$hash = hash_pbkdf2('sha1', 'password', 'salt', 1, 0);
$rawHash = hash_pbkdf2('sha1', 'password', 'salt', 1, 0, true);

var_dump($hash);
var_dump(bin2hex($rawHash));

Expected result:
----------------
string(40) "0c60c80f961f0e71f3a9b524af6012062fe037a6"
string(40) "0c60c80f961f0e71f3a9b524af6012062fe037a6"

Actual result:
--------------
string(20) "0c60c80f961f0e71f3a9"
string(40) "0c60c80f961f0e71f3a9b524af6012062fe037a6"