$buf = preg_replace('#downloads\.php\?cat_id=([0-9]*?)(\'|")#si', 'modai-kategorija-\1.html\2', $buf);

$buf = preg_replace('#downloads\.php\?cat_id=([0-9]*?)(&|&)download_id=([0-9]*?)(\'|")#si', 'modai-kategorija-\1-failas-\3.html\4', $buf);