Just completed the research on an upcoming migration of AWS RDS database from Aurora MySQL 5.7-compatible to latest 8.0-compatible. All went well but there may be some issues when using old versions of PHP with old mysqli extensions.
Problem was at connection, the following error was returned:
Connect Error (2054) Server sent charset unknown to the client. Please, report to the developers
Some research gave me that MySQL 8.0 servers by default use utf8mb4 charset, causing issues for older versions of mysqli.
The solution is either to upgrade the client or configure the server to use utf8 charset instead. With AWS RDS this is managed by setting the following cluster parameters:
"character_set_client": "utf8",
"character_set_connection": "utf8",
"character_set_database": "utf8",
"character_set_server": "utf8"
Following the AWS recommendations to always create a separate parameter group, this is easy to add. A reboot of the server is required so the parameters are applied.