As I wrote the following code I found the strong situation in imap_expunge(). It will expunge mails with odd number only but not every mail as I expected.

$mbox =  imap_open("{localhost/pop3:110INBOX", "test", "test");

$num_msg = imap_num_msg($mbox);
echo "start at $num_msg...<br>";
for ( $i=1; $i<=$num_msg; $i++ ) {

   imap_delete($mbox, $i);
   imap_expunge($mbox);
   echo "doing $i....<br>";
}
imap_expunge($mbox);
$num_msg = imap_num_msg($mbox);
echo "now is $num_msg<br>";
imap_close($mbox);

I have the following output:

start at 10...
doing 1....
doing 2....
doing 3....
doing 4....
doing 5....
doing 6....
doing 7....
doing 8....
doing 9....
doing 10....
now is 5

If I modify the code as following the problem solved:

for ( $i=1; $i<=$num_msg; $i++ ) {
   imap_delete($mbox, $i);
   echo "doing $i....<br>";
}
imap_expunge($mbox);

The result is coreect and all mail has been deleted.
This bug occurs if a web mail system with a pop server try to delete lots of spam at a time.