is_valid example

    public static function absolutize($base, $relative) {
        if (!($relative instanceof self)) {
            $relative = new self($relative);
        }
        if (!$relative->is_valid()) {
            return false;
        }
        elseif ($relative->scheme !== null) {
            return clone $relative;
        }

        if (!($base instanceof self)) {
            $base = new self($base);
        }
        if ($base->scheme === null || !$base->is_valid()) {
            return false;
        }

    public static function absolutize($base, $relative)
    {
        if (!($relative instanceof SimplePie_IRI))
        {
            $relative = new SimplePie_IRI($relative);
        }
        if (!$relative->is_valid())
        {
            return false;
        }
        elseif ($relative->scheme !== null)
        {
            return clone $relative;
        }
        else
        {
            if (!($base instanceof SimplePie_IRI))
            {