You declare [font=monospace]$log_username[/font] as an empty string, so, if you see no output, it would seem that your user simply isn't logged in.
I would suspect that this:
if($user_ok = true){
…is a typo. Did you intend to check whether $user_ok is true? You need to use == or ===. A single = is assignment (you're making $user_ok true).
Another thing I would be concerned about is how you're handling passwords. There is never any reason to store the user's password in the session (and there are plenty of reasons not to). The way you are checking the password against your database suggests you might be storing it in plain text, which is also a very bad idea. Passwords should never be stored anywhere in plain text, ever. Hash them first. Then, store them someplace safe, and don't store them anywhere else.