mysqladmin Ver 8.42 Distrib 5.5.24, for Linux on x86_64
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Server version 5.5.24-cll
Protocol version 10
Connection Localhost via UNIX socket
UNIX socket /var/lib/mysql/mysql.sock
Uptime: 7 hours 22 min 25 sec
Threads: 2 Questions: 210642 Slow queries: 7636 Opens: 995 Flush tables: 1 Open tables: 436 Queries per second avg: 7.935
Ok, so you really are running MySQL 5.5.24.
However, the info.php from a domain on the server says it's version "5.1.59".
CentOS 6 comes with MySQL 5.1 by default, so that's the version of the MySQL client
library that all CentOS 6 RPM packages depend on. For this reason the ART MySQL packages install the MySQL 5.1 client library on CentOS 6. This is why you have mysqlclient16-5.1.59-2.el6.art.x86_64 installed.
You could recompile PHP against MySQL 5.5, but because of the compatibility package you don't need to and I'd recommend being happy about that. Rest assured that your MySQL server
is version 5.5.
And Plesk > Tools & Settings > Server Components say it's version "5.1.61-1.el6_2.1".
CentOS 6 currently ships MySQL 5.1.61. I guess this is stale information that was not up-to-date when you viewed it. Plesk might not update this info in realtime, maybe only daily or something. Does Plesk still say MySQL 5.1.61 is installed?