Custom site login using google apps (company domain) emails

I have developed a custom site application that I am hosting on my server. I used php and mysql for making it. It consists of several forms, and some reports based on these forms. I wish to have a login for these forms using our company google apps emails. Also, I wish to have the email address of the user logged in as a php variable so I can save it to the mysql table once a user submits a form. I already have a header and a footer. I just need a simple login box for authentication to add in my index page, and a link to use for log-out. The logout should then take the user back to the login screen.