In private.php, find
Code:
$DB_site->query("INSERT INTO privatemessage (privatemessageid,userid,touserid,fromuserid,title,message,dateline,showsignature,iconid,messageread,folderid) VALUES (NULL,$touserinfo[userid],$touserinfo[userid],$bbuserinfo[userid],'".addslashes(htmlspecialchars($title))."','".addslashes($message)."',".time().",'$signature','$iconid',0,0)");
Directly below it add
Code:
$DB_site->query("UPDATE user SET posts=posts+1");
Save and upload to your site.