php web mail应用程序，如何唯一标识它们

I'm currently building a php scripts which fetches the email from a server using imap functions and stores the details in the database.

My problem is I dont know how to identify new mails from old mails that already exists. and how to get reply mails sent to the mail

use the UID message to determine the last message, you have to store the last UID in the table

  $uidsArray = imap_sort($imapConnection, SORTARRIVAL, 1, SE_UID);
  if ($uidsArray) {
      // read UID last message, XEmailUID - table(mailbox, lastuid mailbox)
      $lastUIDObject = new XEmailUID();
      $lastUIDObject->setImap($mailbox->getId().'/'.$mailboxRef);
      if (!$lastUIDObject->select()) {
           $lastUIDObject->insert();
      }

      $uidMax = 0;

      foreach ($uidsArray as $uid) {
        if ($uid < $lastUIDObject->getUid()) {
           continue;
        }

        if ($uid >= $uidMax) {
            $uidMax = $uid;
        }

       // your function
       $this->_readIMAPMessage(
          $imapConnection,
          $uid,
          $mailboxRef
       );
      }

      if ($uidMax > 0) {
         $lastUIDObject->setUid($uidMax);
         $lastUIDObject->update();
      }
   }