I stuck in a small problem. I want to login from a third party provider. I have a PHP script and set a database behind where user credentials are stored, so the user access the external site by a link and there the user can login to the central service, afterwords the user get redirected back to the website where it comes from. At that point the application/website where the user comes from could access over a one time code the user credentials and use it to login into the website. My problem now is, how to login to wordpress with the login credentials from my php application. The idea is, if the user use it the first time, than a separate script will create a entry in the database table, what seems easier, but still Im missing how to login on wp-login.php if I have the login credentials. As I know GET method is a very bad idea for this. Hope you can help.

Thanks