For local development I use Laravel Valet. Recently, the brew packages have updated to MySQL 8 which changed a few things about its user management. One thing I continue to run into is this error when working with existing Laravel applications.
SQLSTATE[HY000]  The server requested authentication method unknown to the client
So, here's the fix. You can create a user with the "old" authentication mechanisme, which the MySQL database driver for PHP still expects.
CREATE USER 'ohdear_ci'@'localhost' IDENTIFIED WITH mysql_native_password BY 'ohdear_secret'; GRANT ALL PRIVILEGES ON ohdear_ci.* TO 'ohdear_ci'@'localhost';
If you already have an existing user with permissions on databases, you can modify that user instead.
ALTER USER 'ohdear_ci'@'localhost' IDENTIFIED WITH mysql_native_password BY 'ohdear_secret';
After that, your PHP code can once again connect to MySQL 8.