Bug inside PHP CLI sessions View as Markdown

My apologies. I made a mistake in the minimal examples to reproduce:

1. session_id($id); session_start(); echo session_id(); // empty!

2. session_id($id); echo "b"; session_start(); echo session_id(); // FINE!!

3. echo "a"; session_id($id); echo "b"; session_start(); echo session_id();
   // empty again!

Sincerely,
Greg Magarshak
http://qbix.com/invest

I am using my PHP framework inside my PHP scripts, which has code designed
to work across multiple environments, including web requests and CLI
scripts, and uses handlers which extends SessionHandler. I also
tried implements SessionUpdateTimestampHandlerInterface and validateId.
Unfortunately, the session handling in PHP has a trap door internally, that
was likely motivated by not outputting headers after starting to send HTTP
response body.

It's mystical.

1. session_id($id); session_start(); echo session_id(); // empty!
2. session_id($id); echo "Session started.\n"; session_start(); echo
   session_id(); // FINE!!
3. session_id($id); echo "Session started.\n"; session_start(); echo
   session_id(); // empty again!

I can't believe it is there in 2025.

I tried this with models trained on PHP documentation, and asked it to
provide a link explaining this behavior. This is what it said:

I’m going to be very careful here and not hallucinate citations,
because after checking the PHP manual, RFCs, and known bug threads, there
is no official documentation that explains this exact behavior in CLI.
There is also no single bug report that spells it out end-to-end the
way you’ve reverse-engineered it.

So here is the honest answer, split into what is documented, what is
observable in source, and what is not fixable from userland.

Short answer (truthful)

There is no supported way to “fix” this in PHP userland.

What you are seeing is a consequence of how ext/session couples
session-ID finalization to header emission state, and that logic cannot
be made reliable in CLI, because:

CLI has no real headers

session_id() resets the session module’s internal header bookkeeping

Output is tracked via a single edge-triggered latch

That latch is not re-entrant and not monotonic

Earlier output poisons later output

This is by design, not a bug PHP intends to fix.

Let’s explain your three cases precisely Case 1 — fails

self::id($id);session_start();echo session_id(); // empty

Why:

session_id($id) resets session state

No output has occurred since that reset

PHP believes headers are still possible

In CLI, that means “wait forever”

Session ID never commits

---

Case 2 — works

self::id($id);echo "Session started.\n";session_start();echo session_id(); // OK

Why:

session_id($id) resets session state

Output occurs after the reset

Header latch flips to “headers impossible”

session_start() commits the ID

---

Case 3 — fails again

echo 'a';self::id($id);echo "Session started.\n";session_start();echo session_id(); // empty

Why this feels impossible (but isn’t):

echo 'a' flips the latch

session_id($id) clears the latch

Critically: the latch is edge-triggered, not level-triggered

The second echo does not re-arm it

session_start() still thinks headers are possible

This is the part that feels mystical, but it’s consistent.

The deep reason (this is the core insight) The session module tracks “output
since last reset”, not “output happened”

Internally, the logic is effectively:

if (output_has_occurred_since_last_session_reset) {
commit_session_id();
} else {
defer();
}

And:

session_id() is a session reset

Output before that reset is discarded

Output after that reset must be the first edge

Further output does not help

This logic exists because, in HTTP:

Changing the session ID means cookies may change

Cookies are headers

Header logic must be reset when the ID changes

That logic leaks into CLI unchanged.

Sincerely,
Greg Magarshak

https://76183cf5.streak-link.com/CsqFLR9dXbkQa2qdJQNqfbB-/http%3A%2F%2Fqbix.com%2Finvest

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