Newbie question re: MySQL calls w/PHP

[Anon]

I'm looking at a program that does an include of a config file, and that config file makes a MySQL call that looks kinda like this:

$sql = SELECT * from "$prefix"._config;

Can someone explain to me what's happened here? I can't find the definition for 'prefix', and don't really know what that ._ notation means. I can't find it in my 'Core PHP' book either.

Thanks

[Anon]

someone has several tables and they are named as so:

prefix1_config
prefix2_config
prefix3_config
etc...

it is just "dynamic" table-naming