Run a mySQL query as a cron job?

It depends on what runs cron on your system, but all you have to do to run a php script from cron is to do call the location of the php installation followed by the script location. An example with crontab running every hour:

# crontab -e
00 * * * * /usr/local/bin/php /home/path/script.php

On my system, I don't even have to put the path to the php installation:

00 * * * * php /home/path/script.php

On another note, you should not be using mysql extension because it is deprecated, unless you are using an older installation of php. Read here for a comparison.

I personally find it easier use MySQL event scheduler than cron.

Enable it with

SET GLOBAL event_scheduler = ON;

and create an event like this:

CREATE EVENT name_of_event
ON SCHEDULE EVERY 1 DAY
STARTS '2014-01-18 00:00:00'
DO
DELETE FROM tbl_message WHERE DATEDIFF( NOW( ) ,  timestamp ) >=7;

and that's it.

Read more about the syntax here and here is more general information about it.

Try creating a shell script like the one below:

#!/bin/bash

mysql --user=[username] --password=[password] --database=[db name] --execute="DELETE FROM tbl_message WHERE DATEDIFF( NOW( ) ,  timestamp ) >=7"

You can then add this to the cron