What is the absolute latest MySql client library version

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http://www.linuxquestions.org – I run a Debian LAMP server. I've been using the default MySql package a very long time. Currently I've been reading about MariDB. So I replaced mysql-server with mariadb-server. Simply to try it out. There are two version of mariaDB for Debian. They are 10.0 and 5.5. The default Debian MySql client version is 5.5 so naturally I implemented MariaDB 5.5. That way my server version and my client version both match. (5.5.31) This got me thinking. I've never really thought about the MySql client version before. What exactly is it? Is it a specific package or is it a package of PHP. What exactly (HowTos)