Prerequisite: As GMail uses ssl so php must be compiled with ssl support.
Enable POP: Login your Gmail account ->Settings-> Forwarding and POP -> Enable POP(Yes)

Now fill the fields in the form.

Mail Server Address: pop.gmail.com
Mail Server Protocol: pop3
Mail Server Port: 995
User Name: Your gmail account
Password:   Your gmail account password

Click Show Advanced:
Use SSL: Yes (checked)