public static function copy($source, $target, $ignore = null)
    {
        $source = rtrim($source, '\\/');
        $target = rtrim($target, '\\/');

        if (!is_dir($source)) {
            throw new \RuntimeException('Cannot copy non-existing folder.');
        }

        // Make sure that path to the target exists before copying.
        self::create($target);

        $success = true;

        // Go through all sub-directories and copy everything.
        $files = self::all($source);
        foreach ($files as $file) {
            if ($ignore && preg_match($ignore, $file)) {
                continue;
            }
            $src = $source .'/'. $file;
            $dst = $target .'/'. $file;

            if (is_dir($src)) {
                // Create current directory (if it doesn't exist).
                if (!is_dir($dst)) {
                    $success &= @mkdir($dst, 0777, true);
                }
            } else {
                // Or copy current file.
                $success &= @copy($src, $dst);
            }
        }

        if (!$success) {
            $error = error_get_last();
            throw new \RuntimeException($error['message'] ?? 'Unknown error');
        }

        // Make sure that the change will be detected when caching.
        @touch(dirname($target));
    }