It looks like the class you have found is quite old, and is using features that have been removed from current PHP implementations. It's probably not worth trying to fix this class.
However, you can use the built-in imap
functions to retrieve mail from a POP3 mailbox. The reference is here
From the manual, you can open a POP3 mailbox like this (note the /pop3 flag):
// To connect to a POP3 server on port 110 on the local server, use:
$mbox = imap_open ("{localhost:110/pop3}INBOX", "user_id", "password");