Magento CSS/JS Auto Versioning

config.xml

<?xml version="1.0"?>
<!--
/**
 * @category  Interactiv4
 * @package   Interactiv4_MergeTimestamp
 * @copyright Copyright (c) 2015 Interactiv4 SL. (http://www.interactiv4.com)
 *
 * @author      Oscar Recio (@osrecio)
 */ 
-->
<!-- File: app/code/local/Interactiv4/MergeTimestamp/etc/config.xml -->
<config>
    <modules>
        <Interactiv4_MergeTimestamp>
            <version>0.1.0</version>
        </Interactiv4_MergeTimestamp>
    </modules>
    <global>
        <models>
            <i4mergetimestamp>
                <class>Interactiv4_MergeTimestamp_Model</class>
            </i4mergetimestamp>
            <core>
                <rewrite>
                    <design_package>Interactiv4_MergeTimestamp_Model_Core_Design_Package</design_package>
                </rewrite>
            </core>
        </models>
        <helpers>
            <i4mergetimestamp>
                <class>Interactiv4_MergeTimestamp_Helper</class>
            </i4mergetimestamp>
        </helpers>
    </global>
</config>

Data.php

<?php
/**
 * @category  Interactiv4
 * @package   Interactiv4_MergeTimestamp
 * @copyright Copyright (c) 2015 Interactiv4 SL. (http://www.interactiv4.com)
 *
 * @author      Oscar Recio (@osrecio)
 */ 
 #File: app/code/local/Interactiv4/MergeTimestamp/Helper/Data.php
class Interactiv4_MergeTimestamp_Helper_Data extends Mage_Core_Helper_Abstract {

}

Interactiv4_MergeTimestamp.xml

<?xml version="1.0"?>
<!--
/**
 * @category    Interactiv4
 * @package     Interactiv4
 * @copyright   Copyright (c) 2015 Interactiv4 SL. (http://www.interactiv4.com)
 *
 * @author      Oscar Recio (@osrecio)
 */
-->
<!-- File: app/etc/modules/Interactiv4_MergeTimestamp.xml -->
<config>
    <modules>
        <Interactiv4_MergeTimestamp>
            <active>true</active>
            <codePool>local</codePool>
        </Interactiv4_MergeTimestamp>
    </modules>
</config>

Package.php

<?php
/**
 * @category  Interactiv4
 * @package   Interactiv4_MergeTimestamp
 * @copyright Copyright (c) 2015 Interactiv4 SL. (http://www.interactiv4.com)
 *
 * @author      Oscar Recio (@osrecio)
 */ 
 #File: app/code/local/Interactiv4/MergeTimestamp/Model/Core/Design/Package.php
class Interactiv4_MergeTimestamp_Model_Core_Design_Package extends Mage_Core_Model_Design_Package
{
    /**
     * Get the timestamp of the newest file
     *
     * @param array $srcFiles
     * @return int $timeStamp
     */
    protected function getNewestFileTimestamp($srcFiles) {
        $timeStamp = null;
        foreach ($srcFiles as $file) {
            if(is_null($timeStamp)) {
                //if is first file, set $timeStamp to filemtime of file
                $timeStamp = filemtime($file);
            } else {
                //get max of current files filemtime and the max so far
                $timeStamp = max($timeStamp, filemtime($file));
            }
        }
        return $timeStamp;
    }


    /**
     * Merge specified css files and return URL to the merged file on success
     *
     * @param $files
     * @return string
     */
    public function getMergedCssUrl($files)
    {
        // secure or unsecure
        $isSecure = Mage::app()->getRequest()->isSecure();
        $mergerDir = $isSecure ? 'css_secure' : 'css';
        $targetDir = $this->_initMergerDir($mergerDir);
        if (!$targetDir) {
            return '';
        }

        // base hostname & port
        $baseMediaUrl = Mage::getBaseUrl('media', $isSecure);
        $hostname = parse_url($baseMediaUrl, PHP_URL_HOST);
        $port = parse_url($baseMediaUrl, PHP_URL_PORT);
        if (false === $port) {
            $port = $isSecure ? 443 : 80;
        }

        //get timestamp of newest source file
        $filesTimeStamp = $this->getNewestFileTimestamp($files);

        // merge into target file
        $targetFilename = md5(implode(',', $files) . "|{$hostname}|{$port}") . "_" . $filesTimeStamp . '.css';

        //If the file with the proper timestamp as part of its filename already exists, there's no reason to check again to see if
        //we need to remerge the css files
        if(!file_exists($targetDir . DS . $targetFilename)) {
            $mergeFilesResult = $this->_mergeFiles(
                $files, $targetDir . DS . $targetFilename,
                false,
                array($this, 'beforeMergeCss'),
                'css'
            );
            if ($mergeFilesResult) {
                return $baseMediaUrl . $mergerDir . '/' . $targetFilename;
            }
        } else {
            return $baseMediaUrl . $mergerDir . '/' . $targetFilename;
        }
        return '';
    }

    /**
     * Merge specified javascript files and return URL to the merged file on success
     *
     * @param $files
     * @return string
     */
    public function getMergedJsUrl($files)
    {
        //get timestamp of newest source file
        $filesTimeStamp = $this->getNewestFileTimestamp($files);

        $targetFilename = md5(implode(',', $files)) . '_' . $filesTimeStamp . '.js';
        $targetDir = $this->_initMergerDir('js');
        if (!$targetDir) {
            return '';
        }

        if ($this->_mergeFiles($files, $targetDir . DS . $targetFilename, false, null, 'js')) {
            return Mage::getBaseUrl('media', Mage::app()->getRequest()->isSecure()) . 'js/' . $targetFilename;
        }
        return '';
    }

}