code alteration

how do I alter the below code to have it count the posts in forumid 1 and 5? thanks in advance

$grabthreadids=$DB_site->query("SELECT threadid FROM post WHERE post.userid='$post[userid]' AND post.visible=1");

$post[thisforumonly] = "0";

while ($getcount=$DB_site->fetch_array($grabthreadids)) {
$checkforumid=$DB_site->query_first("SELECT forumid FROM thread WHERE threadid=$getcount[threadid] AND thread.visible=1");
if ($checkforumid[forumid] == $forum[0]) {
$post[thisforumonly]++;
}
}