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Answer 1

How can I determine what is the mechanism which causes session_start to create new sessions wrather than resume a previous one?

This is visible in the PHP sourcecode for the session_start function. You need to read the C-code and compare with your usage.

From what I know about sessions, session_start won’t start a new session if already one is active. To find out if a session is already active, please see How to tell if a session is active?.

However if a session is started (and it didn’t existed earlier) and then closed and you create a new session in the same request, PHP might think that the session does not exists (because the cookie from the browser is still empty). So then a second, also new, session will be started.

If you’re unsure what does what, just create yourself a test script where you play around with scenarios.

A possible scenario:

1. Browser sends request
2. PHP starts
3. session_start() is called. No session cookie exists, PHP will create a new session id and will create cookie headers.
4. you close the session.
5. session_start() is called. No session cookie exists (in the request), PHP will create a new session id and will create cookie headers.

Two sessions have been created of which one will not be used by the browser for subsequent requests (the session id header for the cookie has been “overwritten” (the last cookie header replaces previous ones for the cookie in question).

To debug things, headers_list can be useful as well as $_COOKIES.