I've asked a question on ServerFault but I (now) suspect DBA may be a better fit. It's shades of gray as usual but now I'm curious.
Should I just repost there and let the question get closed here? Is there a way to "export" a question?
Extraordinarily good questions can be migrated to other Stack Exchange sites - Flag your question for moderator attention if you think it is such an extraordinary question, tell us where you think it belongs, and the moderators will look at it and decide if it's something we're comfortable pitching over the wall.
The worst we'll do is decline your request to migrate the question (and we'll usually tell you why).
You can also ask (politely) in the other site's chat room to see if they (a) want the question, or (b) want to come over here and help us answer it.
If you think we've got our heads up our butts for declining your migration (or don't want to wait for us) and the users on the other site think it's a good fit you can also delete your question here and re-ask it on the other site.
In the case of your MongoDB question I'm not certain DBA is the right place for it -- It may be a Mongo problem, or it may be a PHP glitch.
I'd like to see you do a little more legwork (try using the Mongo command line tools to connect, check the logs from your PHP application and any load-balancing components, make sure virtual IPs are moving over like they should, etc.) before punting this to the DBA folks.