Adds file modified date as query string

/**
 * Adds file modified date as query string and returns URL to given asset.
 * 
 *
 * @param $relPath string File path relative to /site/templates/, such as "assets/css/style.css"
 * @return string URL with cache busting query parameter
 */
function assetUrl($relPath)
{
    $fullPath = wire()->config->paths->templates . $relPath;
    $modified = filemtime($fullPath);

    $url = wire()->config->urls->templates . $relPath . "?v=$modified";
    return $url;
}