A hash_pbkdf2() implementation for PHP versions 5.3 and 5.4.

## pbkdf2.php
<?php

/**
 * Generate a PBKDF2 key derivation of a supplied password
 *
 * This is a hash_pbkdf2() implementation for PHP versions 5.3 and 5.4.
 * @link http://www.php.net/manual/en/function.hash-pbkdf2.php
 *
 * @param string $algo
 * @param string $password
 * @param string $salt
 * @param int $iterations
 * @param int $length
 * @param bool $rawOutput
 *
 * @return string
 */
function compat_pbkdf2($algo, $password, $salt, $iterations, $length = 0, $rawOutput = false)
{
    // check for hashing algorithm
    if (!in_array(strtolower($algo), hash_algos())) {
        trigger_error(sprintf(
            '%s(): Unknown hashing algorithm: %s',
            __FUNCTION__, $algo
        ), E_USER_WARNING);
        return false;
    }

    // check for type of iterations and length
    foreach (array(4 => $iterations, 5 => $length) as $index => $value) {
        if (!is_numeric($value)) {
            trigger_error(sprintf(
                '%s() expects parameter %d to be long, %s given',
                __FUNCTION__, $index, gettype($value)
            ), E_USER_WARNING);
            return null;
        }
    }

    // check iterations
    $iterations = (int)$iterations;
    if ($iterations <= 0) {
        trigger_error(sprintf(
            '%s(): Iterations must be a positive integer: %d',
            __FUNCTION__, $iterations
        ), E_USER_WARNING);
        return false;
    }

    // check length
    $length = (int)$length;
    if ($length < 0) {
        trigger_error(sprintf(
            '%s(): Iterations must be greater than or equal to 0: %d',
            __FUNCTION__, $length
        ), E_USER_WARNING);
        return false;
    }

    // check salt
    if (strlen($salt) > PHP_INT_MAX - 4) {
        trigger_error(sprintf(
            '%s(): Supplied salt is too long, max of INT_MAX - 4 bytes: %d supplied',
            __FUNCTION__, strlen($salt)
        ), E_USER_WARNING);
        return false;
    }

    // initialize
    $derivedKey = '';
    $loops = 1;
    if ($length > 0) {
        $loops = (int)ceil($length / strlen(hash($algo, '', $rawOutput)));
    }

    // hash for each blocks
    for ($i = 1; $i <= $loops; $i++) {
        $digest = hash_hmac($algo, $salt . pack('N', $i), $password, true);
        $block = $digest;
        for ($j = 1; $j < $iterations; $j++) {
            $digest = hash_hmac($algo, $digest, $password, true);
            $block ^= $digest;
        }
        $derivedKey .= $block;
    }

    if (!$rawOutput) {
        $derivedKey = bin2hex($derivedKey);
    }

    if ($length > 0) {
        return substr($derivedKey, 0, $length);
    }

    return $derivedKey;
}

// test
if (realpath($_SERVER['argv'][0]) === __FILE__ && function_exists('hash_pbkdf2')) {
    $password = 'password';
    $savedPassword = 'sha256,salt,65536,4156f668bb31db3a17f4d1b91424ef0d417ad1f35d055aceaebd8da0f6a44b7e';
    list($algo, $salt, $iterations, $hash) = explode(',', $savedPassword);

    var_dump(
        $hash,
        hash_pbkdf2($algo, $password, $salt, $iterations, strlen($hash)),
        compat_pbkdf2($algo, $password, $salt, $iterations, strlen($hash))
    );
}