Session bug?

Sorry if I missed the answer in this NG. Searching hasn't turned up
anything other than permissions issues, which this isn't.

OS: Red Hat 8
PHP: 4.3.3
HTTPD: Apache 2.0.47

In PHP.INI:

session_autostart = 1;
session_use_cookies = 0;

How sessions are maintained:

Visitor hits the page -> if no session_id in the URI, get session_id
and add to all URIs on the page, carrying over to further pages via
query string in URI.

Session Issues:

(1) Even though sessions are being written and stored, I am receiving
many error_log entries:

PHP Warning: Unknown(): The session id contains invalid characters,
valid characters are only a-z, A-Z and 0-9 in Unknown on line 0
PHP Warning: Unknown(): Failed to write session data (files). Please
verify that the current setting of session.save_path is correct (/tmp)
in Unknown on line 0

Now why would PHP write itself a session_id that it couldn't use? I'm
certainly not writing it. Maybe incomplete or mangled URIs are being
written, however I can't tell which ones, because everything is
working fine on the sites, URI-wise...every dynamic link goes where I
want it to and carries over the session_id correctly in the query
string, as described above. In addition (see below), sessions ARE
being written to /tmp (chmod 777). I have since changed it to a more
local directory, but I don't know that I needed to.

(2) It looks like there are several sessions being started
near-simultaneously for each request, however the one that gets
written to the URIs as described above are carrying over and being
used properly.

Why isn't PHP recognizing that the session has already auto_started?
What's happening when PHP launches multiple sessions for a single
request?

I appreciate any insight. Thanks!

James