Filters all CSS url()'s through a callable.
string $content The CSS:
callable $callback A PHP callable:
integer $limit Limit the number of replacements:
integer $count Will be populated with the count:
string The filtered CSS
public static function filterUrls($content, $callback, $limit = -1, &$count = 0) {
return preg_replace_callback(static::REGEX_URLS, $callback, $content, $limit, $count);
}