why php sessions dying? since can't i revive them?

i have an app controlling php paypal api. simple proceed works remuneration web use paypal token following browser route paypal token user pay. after remuneration sum have confirmed, paypal redirects behind url creatively set use call.

this works, millions each day, et cetera.

strange thing is, paypal redirects back, php event gone. it's .

first question: since happening? both pages same domain, both https. event works requests adult until paypal route back.

the related forum thread suggests workaround, insist event id paypal ask following collect after revive session. great, solely doesn't seem work.

i supplement record statements:

log(session_id());

before after several redirects. opening behind paypal, i record more.

log("session id " . session_id());
$session_id = get_session_id_from_paypal();
log("setting behind " . $session_id);
session_id($session_id);
session_start();
log("session id " . session_id());

the outcome during i'd expect:

session_id fc8f459a186a3f4695ff9ac71b563825

sourroundings behind 82460dcf8c8ddd538466e7cb89712e72

session_id 360ba3fd99d233e0735397278d2b2e55

second question: since event id during i set to? am i doing wrong? or, during least, since zero event variables back?