Is there a special library I need to include to use the send mail function. I want users to be able to type their email address in a form, choose subscribe or unsubscribe then click submit and have it send mail doing the appropriate action.

I keep getting the following error: Fatal error: Failed opening required 'mailinglist.php' (include_path='./:/usr/local/lib/php')

mailinglist.php is a file that has the code that I am inserting into my rightcell content in my page.