If you have used a local mysql server and decided to port to an external one, you might have a problem that symfony can not connect to the external db. It can't even get a response from the server.
If you experience that kind of issue, the thing might be that you forgot to include "host=" in your dsn definition.
Notice the difference, which is 'host='. It is also forgotten on docs, not written neither on Practical Smyfony nor Definitive Guide. So you might also forget. It does not make a problem if you simply connect to a local mysql server.