Source

File: wp-includes/class-pop3.php

function last ( $type = "count" ) {
        //  Returns the highest msg number in the mailbox.
        //  returns -1 on error, 0+ on success, if type != count
        //  results in a popstat() call (2 element array returned)

        $last = -1;
        if(!isset($this->FP))
        {
            $this->ERROR = "POP3 last: " . _("No connection to server");
            return $last;
        }

        $reply = $this->send_cmd("STAT");
        if(!$this->is_ok($reply))
        {
            $this->ERROR = "POP3 last: " . _("Error ") . "[$reply]";
            return $last;
        }

        $Vars = preg_split('/\s+/',$reply);
        $count = $Vars[1];
        $size = $Vars[2];
        settype($count,"integer");
        settype($size,"integer");
        if($type != "count")
        {
            return array($count,$size);
        }
        return $count;
    }