Filters all references -- url() and "@import" -- through a callable.
string $content The CSS:
mixed $callback A PHP callable:
string The filtered CSS
protected function filterReferences($content, $callback, $limit = -1, &$count = 0) {
$content = $this
->filterUrls($content, $callback, $limit, $count);
$content = $this
->filterImports($content, $callback, $limit, $count, false);
$content = $this
->filterIEFilters($content, $callback, $limit, $count);
return $content;
}