Expiry

A currency created with a lifetime — new Currency('stars', 'P1Y') — makes each credit to a member expire that long after its moment: points earned, granted, refunded or received. A currency without one never expires.

First in first out

Lots follows what is left of each credit, the movements applied in the order the ledger wrote them:

• a debit that names a credit — the reversal of an earning, the expiry of a credit — takes from that credit first;
• every debit then takes from the oldest credits still alive at its moment, then from the oldest of the others;
• what no credit covers is a debt, which the next credits pay before they open a lot of their own.

Expiring

$points->expire($member);          // the default currency
$points->expire($member, 'stars');
$points->expireAll();              // every member, every currency with a lifetime

Each credit whose lifetime has ended with points left is expired by a movement #expire:<credit key> to expiry:expired, dated at the end of its lifetime. Expiring again finds nothing more to expire. expireAll() is the task a scheduler runs; it looks at the members credited before the cutoff of each currency, one by one. A spending and a transfer expire the payer's credits first, so a member never spends points that have already expired.