Basic asset cache busting plugin for Craft CMS.

HelpersPlugin.php

<? namespace Craft;

# craft/plugins/helpers/HelpersPlugin.php

class HelpersPlugin extends BasePlugin
{
  public function getName()
  {
    return Craft::t('Helpers');
  }

  public function getVersion()
  {
    return '1.0';
  }

  public function getDeveloper()
  {
    return 'oof. Studio';
  }

  public function getDeveloperUrl()
  {
    return 'http://oof.studio/';
  }
}

HelpersVariable.php

<? namespace Craft;

# craft/plugins/helpers/variables/HelpersVariable.php

class HelpersVariable
{
  /*
   * Get the text content of an asset in the public directory
   */
  public function inlineAsset(String $asset)
  {
    $file = $this->releasePath() . 'public/assets/' . $asset;
    return IOHelper::getFileContents($file);
  }

  /*
   * Dynamically fetch the app's root directory
   */
  public function releasePath()
  {
    return str_replace('craft/plugins/helpers/variables', '', __DIR__);
  }

  /*
   * Get the last modified date for a static asset
   */
  public function assetModificationTime(String $asset)
  {
    $file = $this->releasePath() . '/public/' . $asset;
    return IOHelper::getLastTimeModified($file)->getTimestamp();
  }

  /*
   * Get a public URL for an asset in the public directory
   */
  public function assetUrl($asset, $params = [])
  {
    return UrlHelper::getUrlWithParams($asset, $params);
  }

  /*
   * Get the filesystem path for an uploaded file
   */
  public function assetPath(AssetFileModel $asset)
  {
    $source = $asset->getSource();
    if  ($source->type != 'Local')
    {
      throw new Exception(Craft::t('Paths not available for non-local asset sources'));
    }

    $sourcePath = $source->settings['path'];
    $folderPath = $asset->getFolder()->path;

    return $sourcePath . $folderPath . $asset->filename;
  }

  public function includeCss($asset)
  {
    $mtime = $this->assetModificationTime($asset);
    $url = $this->assetUrl($asset, ['mtime' => $mtime]);
    craft()->templates->includeCssFile($url, true);
  }

  public function includeJs($asset)
  {
    $mtime = $this->assetModificationTime($asset);
    $url = $this->assetUrl($asset, ['mtime' => $mtime]);
    craft()->templates->includeJsFile($url);
  }
}

template.twig

{# Add ?mtime=1234567890 to the asset url: #}
{{ craft.helpers.includeCss('/assets/css/app.css') }}