I use PHP imap with imap_open. You will find the doc here
Simple usage:
$stream = imap_open ("{localhost:110/pop3}INBOX", "user_id", "password");
$mbox = imap_check($stream);
$msg_amount = $mbox->Nmsgs;
It is easy to use and works good but it will getting slow if you try to fetch 5000+ mails.
Alternative you can use fsockopen.
Edit:
To install:
To get these functions to work, you have to compile PHP with
--with-imap[=DIR] , where DIR is the c-client install prefix. From our
example above, you would use --with-imap=/usr/local/imap-2000b . This
location depends on where you created this directory according to the
description above. Windows users may include the php_imap.dll DLL in
php.ini. IMAP is not supported on systems earlier than Windows 2000.
This is because it uses encryption functions in order to enable SSL
connections to the mail servers.