Piwik\AssetManager\UIAssetMerger::isFileUpToDate PHP Method

    private function isFileUpToDate()
    {
        $f = fopen($this->mergedAsset->getAbsoluteLocation(), 'r');
        $firstLine = fgets($f);
        fclose($f);
        if (!empty($firstLine) && trim($firstLine) == trim($this->getCacheBusterValue())) {
            return true;
        }
        // Some CSS file in the merge, has changed since last merged asset was generated
        // Note: we do not detect changes in @import'ed LESS files
        return false;
    }