How do I login as a user from a php file on the same server?

[gronk]

I was trying to send login cookies from an external login_me.php file (without knowing password)? How do I actually do this? Is there an API for this?
What file do I have to include in my php file to be able to authenticate an user?
Thanks

[SKaero]

If you don't know the user password you wont be able to login.