MySQL configuration: when to use hyphen and when to use underscore

According to the suggestion in the mysql bug report discussing the confusing state of the mysql docs:

1. The my.cnf and command line options you can use either _ or - within the option name.
2. Server variables are _ only when working with the MySQL client (modifying from SET for example).

In summary, you can use _ and - interchangeably within a config file.

No.

It's just one of those things that happen when a team of people design a product.
Different conventions get used leading to inconsistency.

See here: http://dev.mysql.com/doc/refman/5.5/en/mysqld-option-tables.html

And scroll to the options starting with ssl, you'll see what I mean.