How to add a new field in php using plugin

Question

How to use plugin facility to add a new field on squirrelmail.what files need to change to add a field using plugin in php.

is somebody thewre for guidence and having step by step procedure to do this .i need to add a openid field(textbox) using plugin on squirrelmail user interface.there are the files like function.php,setup.php etc. but still not getting what changes i have to made and in what files .Need to learn how to add a new field on squirrelmail user interface using the plugin concept or facility.