Redirect users from authentication URLs to signup page I'm using Drupal 6.

I want to redirect users from /user/login, /user/register, /user to /sign-up if the user is the anonymous user. This page would show both login and register forms together.

The only page left would be /user/password, which form I don't want to have on /sign-up.
I'm comfortable with PHP, but I'm not sure about which hooks should I use to achieve this.

PS. I already know how to print those forms on my own. My doubt is just about the URLs.
There is no dedicated hook for things like this, but you can implement hook_init() and then do the redirect there. Something like this:

function yourmodule_init() {
  global $user;
  if (in_array($_GET['q'], array('user', 'user/login', 'user/register') &amp;&amp; !$user-&gt;uid) {
    drupal_goto('sign-up');
  }
}