adding more player.variables

[Prozac]

let's say i want to add another variable to the player
called player.foo
and the value of foo can be any string integer or array.

how would i go about doing this?
and doing this so that the new player attribute can only be modified
from a serverside command?

i imagine it's something about joining a class or a database npc,
could anyone please point me in the right direction?