Web Site cannot establish connection errors with "Server sent charset unknown"

Symptoms

New server set up with MySQL 8 and sites transferred give cannot establish connection errors and the PHP error_log shows "PHP Warning: mysqli_connect(): (HY000/2054): Server sent charset unknown to the client" is because the server collation and charset is incorrect.

Description 

The following command

mysql -e "show variables like 'char%';"

 may give output like the following:

+--------------------------+--------------------------------+
| Variable_name            | Value                          |
+--------------------------+--------------------------------+
| character_set_client     | utf8mb4                        |
| character_set_connection | utf8mb4                        |
| character_set_database   | utf8mb4                        |
| character_set_filesystem | binary                         |
| character_set_results    | utf8mb4                        |
| character_set_server     | utf8mb4                        |
| character_set_system     | utf8                           |
| character_sets_dir       | /usr/share/mysql-8.0/charsets/ |
+--------------------------+--------------------------------+

and databases with charset latin1 or utf8 may not work. The MySQL server collation and charset would need to be changed.

Workaround

The collation and charset were changed by modifying /etc/my.cnf and under [mysqld] and adding:

collation-server = utf8_unicode_ci
character-set-server = utf8

and then afterward add:

[client]
default-character-set=utf8

[mysql]
default-character-set=utf8

and then restart MySQL.

/scripts/restartsrv_mysql

Then when you check collation and charset again you should have:

mysql -e "show variables like 'char%';"
+--------------------------+--------------------------------+
| Variable_name            | Value                          |
+--------------------------+--------------------------------+
| character_set_client     | utf8                           |
| character_set_connection | utf8                           |
| character_set_database   | utf8                           |
| character_set_filesystem | binary                         |
| character_set_results    | utf8                           |
| character_set_server     | utf8                           |
| character_set_system     | utf8                           |
| character_sets_dir       | /usr/share/mysql-8.0/charsets/ |
+--------------------------+--------------------------------+