session_start() → POST遷移 → history.back 等で戻ると「有効期限切れ」って出てしまうのは、session_start()時に以下ヘッダが勝手に出力されてクライアントキャッシュが無効になってしまうため。 Expires: Thu, 19 Nov 1981 08:52:00 GMT Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0 Pragma: no-cache
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