PHP Session: check without starting one - php

Question

Is it possible to check if a certain session ID exists without starting a new one?
The reasons I'm asking this are:
If every user not logged in visiting the site requires session_start(), soon the session folder will be bloated with unnecessary files (even with garbage collector cleaning up expired sessions).
Of course this can be mitigated by destroying the sessions of such users. However, in this case, it seems there would be an overhead of creating and destroying files (unless PHP is smart enough to cancel these operations). Is this a bad practice?
If the user is just a guest and doesn't require a session, there is no need to create one for him.
Ideal Solutions:
if( session_exist($_COOKIE['PHPSESSID']) )
{
session_start();
}
Or:
session_start(RESUME_ONLY);
Follow-up
There is a PHP package that explains much better the aforementioned issue. Briefly, some interesting excerpts are:
Resuming A Session
[...]
You could start a new session yourself to repopulate $_SESSION, but that will incur a performance overhead if you don't actually need the session data. Similarly, there may be no need to start a session when there was no session previously (and thus no data to repopulate into $_SESSION). What we need is a way to start a session if one was started previously, but avoid starting a session if none was started previously.
[...]
If the cookie is not present, it will not start a session, and return to the calling code. This avoids starting a session when there is no $_SESSION data to be populated.
Source: Aura.Auth
Notes:
This question was initially posted as Checking for PHP session without starting one?.
However, IMO, none of the answers addressed properly the issue.

Answer 1

I think you are trying to solve the wrong problem here. To answer your question, no, you must use session_start before preforming session operations. You could also destroy the session if it isn't needed in the same request with session_destroy.
You can work around the session_start problem with some clever session storage options, but I think that is just the wrong answer to the problem you have. The size of sessions stored on the FS is really negligible when your servers installation footprint is at least 8GB. Furthermore, the default session storage location is /tmp which is commonly setup as a ramfs to avoid any disk overhead. If you are running into the limitations of FS session storage, you probably need to use a different SessionHandler like memcached, not worrying about if you need to start a session. On a related note, stop prematurely optimizing your code. Session storage is not going to be an issue until you need more than one server. In reality there are going to be many more database queries that would benefit from an additional index than obscure optimizations like this one.