$cat help….

I read over on https://www.chrisjdavis.org/index.php/2004/07/15/faking-it-wordpress-as-cms/551/2/ and also on many other hack pages, that by putting
$cat="2" ;
in your header that page will only display posts from that category, I desperately need to be able to do this for alot of my pages, please can someone explain to me how to do it, as when I have done that there is no change.
I have tried this as well as many “One Category” hacks etc.
Please please help