The new cluster runs PHP 5.6 by default, and PHP 7.1 with suPHP (instructions to follow). Testing old apps has revealed two specific issues:

1. Warning: mysql_connect(): mysqlnd cannot connect to MySQL 4.1+ using the old insecure authentication.
2. Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead

We aren't sure why, but you may get one or the other or both messages. It *appears* that if you fix problem #2 both problems go away, but if you only fix problem #1, you'll be left with problem #2. So, at a bare minimum, solve problem #2.

Here's how to address each problem:

1. Log in to phpMyAdmin as the db user required by your PHP script and run the following SQL in the context of the app's DB:
   SET SESSION old_passwords=0;
SET PASSWORD=PASSWORD('my_password');
   This has worked without further effort, but some say that you should also run:
   FLUSH PRIVILEGES;
   as the DB admin user

2. You can either fix the problem by changing your MySQL Extension methods (mysql_*) to MySQL Improved Extension methods (mysqli_*), or the sub-optimal "ignore the deprecation" method of adding error_reporting(E_ALL ^ E_DEPRECATED); to the head of your scripts.