You may have noticed that there exists a File class in the core (cake/libs/file.php). But how can you use it? Let me give you a simple example:

// app/controllers/test_controller.php

// includes cake/libs/file.php
uses('file');

class TestController extends AppController
{
    var $uses = null;

    function index()
    {
        $f = new File(TMP.'test.txt', true);
        $f->append('hello');

        exit;
    }
}

In a similar way you can use the other classes available in the cake/libs folder in your application.