从最新的线程php删除旧电子邮件

I have the following structure:

我有以下结构:

$email = array(
      4 => array(
          4,  //contains email messages from uids 4 + 3 + 2
          3,  //contains email messages from uids 3 + 2
          2   //contains email message from uid 2
));
$em