passing a variable through if statement

i need help passing a variable through an if statement...

the link im using is:
echo "<a href="$PHP_SELF?cat">$name</a> ($numrows)<BR>";

elseif($_SERVER['QUERY_STRING'] == "cat") {
include('cat.php');
}

how can i pass a variable like $cat_id through this if statement and show up for cat.php?

it works fine if i use a link straight to cat.php like:
echo "<a href="$PHP_SELF?cat_id=$cat_id">$name</a> ($numrows)<BR>";