php安全的strip_tags

function safe_strip_tags($s, $keeps = null) // $keeps = 'img|span|/span'
{
    if (!$keeps) {
        return preg_replace('#<[^<>]+>#is', '', $s);
    } else {
        return preg_replace('#<(?!'.$keeps.'|[^A-Za-z/])[^<>]*>#is', '', $s);
    }
}

echo strip_tags('<latex> ab<0 </latex>', 'img|span|/span'); // ab
echo safe_strip_tags('<latex> ab<0 </latex>', 'img|span|/span'); // ab<0