how to download mails attachment to a specific folder using IMAP and php

To save attachments as files, you need to parse the structure of the message and take out all parts that are attachments on it's own (content disposition). You should wrap that into classes of their own so you have an easy access and you can handle errors more easily over time, email parsing can be fragile:

$savedir = __DIR__ . '/imap-dump/';

$inbox = new IMAPMailbox($hostname, $username, $password);
$emails = $inbox->search('ALL');
if ($emails) {
    rsort($emails);
    foreach ($emails as $email) {
        foreach ($email->getAttachments() as $attachment) {
            $savepath = $savedir . $attachment->getFilename();
            file_put_contents($savepath, $attachment);
        }
    }
}

The code of these classes is more or less wrapping the imap_... functions, but for the attachment classes, it's doing the parsing of the structures as well. You find the code on github. Hope this is helpful.

Although using PHP + Cron and a standard mail server might work, the amount of work needed to handle all the edge cases, error reporting, etc might not be worth the time. Although I haven't used it, Postmark Inbound seems like an incredible (paid) service that will eliminate most of the headache of processing email via the PHP imap api.

If you want to try to handle everything via PHP, you might want to check this resource out.