谢谢大卫的回复。我用这种方法解决了这个问题（我希望前两个函数是自我解释的）：

$memberID = getIDFromMembers($pdo, $email);
$currListID = getIDFromList($pdo, $listname);

// remove record from join table
$sql = 'DELETE FROM `members2lists` WHERE `member_fk` = :member AND `list_fk` = :list';
$stmt = $pdo->prepare($sql);
$stmt->bindParam(':member', $memberID);
$stmt->bindParam(':list', $currListID);
$stmt->execute();

// check if another record exists in the join table for the same member
$sql = 'SELECT `member_fk` FROM `members2lists` WHERE `member_fk` = :member';
$stmt = $pdo->prepare($sql);
$stmt->bindParam(':member', $memberID);
$stmt->execute();
$row = $stmt->fetch();

if($row === false) 
{
  // there isn't, the member is an orphan, so delete
  $sql = 'DELETE FROM `members` WHERE `member_email` = :email';
  $stmt = $pdo->prepare($sql);
  $stmt->bindParam(':email', $email);
  $stmt->execute();
}

我很高兴的意见，从成员对如何改善这一代码！