Server requested authentication method unknown to the client [mysql_old_password]
I had the same error, trying to debug locally some application that connects to remote DBs.
When using php &lt; 5.3, everything worked. 
When using php 5.3 or greater, error shows up.

After hours spent reading and tweaking, i realized this only occurred when connecting as a certain user.
All databases i use have new password length (41).

I changed the password for that user, and everything worked fine, even after changing it again to the previous value.

Hope this helps someone.