MySQL: Server sent charset (255) unknown to the client.

Symptoms

Websites are giving a mysql error, and the log shows "MySQL: Server sent charset (255) unknown to the client."

Description 

With the release of MySQL 8, the default character set was changed to utfmb4. Since this differs than what may be in the mysql database, the error is presented.

Workaround

Updating to a version of PHP newer than 7.4 tends to resolve this.

Alternatively, if you need to use an older version of PHP, the fix involves modifying the /etc/my.cnf file to set the default character set to UT8:

[mysqld]
collation-server = utf8_unicode_ci
character-set-server = utf8

Once modified, restart mysql and the websites should load properly.