PDO::__construct(): Server sent charset (255) unknown to the client. Please, report to the developers

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MySQL 8 changed the default charset to utf8mb4. But some clients don’t know this charset. Hence when the server reports its default charset to the client, and the client doesn’t know what the server means, it throws this error.

See also https://bugs.mysql.com/bug.php?id=71606

That bug is against the MySQL Connector/C++ so it’s affecting more than just PHP.

Okay—I got it to work by changing the character set to utf8, to be compatible with non-upgraded clients. I added this to /etc/my.cnf and restarted mysqld:

[client]
default-character-set=utf8

[mysql]
default-character-set=utf8


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

I found these settings in an answer from 2010: Change MySQL default character set to UTF-8 in my.cnf?

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