Create a Joomla 2.5 Plugin out of a standard PHP class

I have a Joomla 2.5 plugin that is executed when a Joomla user is logged into the website. This plugin currently pulls some information from a database which will be used to create a session cookie for a 3rd party API. (login and password) I have all of the files required for the API (note attached) I would to convert this API into a Joomla plugin so that it can be used when the user logs into Joomla. Specifically, the command: $user_creds = $ohapi->login_module()->login(u,p); This command creates the session cookie for the 3rd party system, however it does not work correctly when executed within my existing plugin. I am aware that the session cookie will not work on a development site (needs to be the proper URL), however I am hoping to execute the command to create the session cookie without an error. Note the file log.php. This is an example of the API which is executed when the user name and password are submitted through the URL. For example: [login to view URL] The u and p are used to create the session cookie. I would like this to happen within the Joomla plugin. if I supply the u and p.