.htaccess redirect with mysql content in url

Now, since the "page-title" part wasn't part of the original URL, and is information from a mysql table that corresponds to the id "123", how can I 301 redirect the old URL's to the new URL's ?

The only rewrite rules you need is to make the nicer looking URL route to view.php, which it sounds like you've already got. Probably looks something like this:

RewriteCond %{REQUEST_FILENANE} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([0-9]+)/([^/]+)\.html /view.php?id=$1 [L]

When that request gets to view.php you can check the $_SERVER['REQUEST_URI'] variable, and it will look like '/123/page-title.html'. When you see that, you know just to serve the content and be done with it.

However, when the view.php looks at $_SERVER['REQUEST_URI'] and it looks like '/view.php?id=123', then you know someone has requested the page http://mysite.com/view.php?id=123 and not the nicer looking URL. When this happens, the view.php needs to look up the page-title from the database using $_GET['id']'s sanitized value, then create a redirect (all this needs to happen before anything gets printed to the browser):

Something like:

    header('HTTP/1.1 301 Moved Permanently');
    header('Location: /' . $id . '/' . urlencode($page-title) . '.html');
    exit();

That'll redirect the browser to the page http://mysite.com/view.php?id=123. You don't need anything else in htaccess.