The new mysqlnd library necessitates the use of MySQL 4.1’s newer 41-byte password format. Continued use of the old 16-byte passwords will cause mysql_connect() and similar functions to emit the error, “mysqlnd cannot connect to MySQL 4.1+ using old authentication.”
If so, see http://stackoverflow.com/a/1340538/187954 for information on updating your password.
http://stackoverflow.com/questions/14612551/mysql-remote-connection-fails-with-unknown-authentication-method