I have below code. But I cannot understand what is meant by $seat_key => $seat in foreach. In other terms, I understand foreach ($seats as $seat), but not foreach ($seats as $seat_key => $seat)
foreach ($seats as $seat_key => $seat) {
    $email_found = false;
    foreach ($email_accounts as $key => $email_account) {
        if (strtolower($email_account->email) == strtolower($seat->email) && empty($email_account->deactivated_since) && empty($email_account->flg_deleted)) {
            unset($seats[$seat_key]);
            unset($email_accounts[$key]);
            $email_found = true;
            if (isset($email_found_seats[$seat->email])) {
                $email_duplicated_seats[] = $seat;
            } else {
                $email_found_seats[$seat->email] = $seat;
            }
        }
    }
}
 
    