"relatively well organized" is the problem. It's written in a fairly unstructured imperative style that's perfectly suitable for short scripts, but even these relatively small pieces of code are outgrowing it. The extensive use of global variables is an obvious thing that I would expect to become a problem soon. It's much less of a problem in php than in most other web frameworks due to the design of handling each HTTP request in a fresh process (so things like the use of $user in both index.php and search.php can't conflict), but it's still quite fragile.

By the standards of 2007 PHP this is good code, and today there's probably lots of worse PHP being written (even if the community as a whole has moved on quite a bit).