How should I get started using PHPUnit as a testing framework for a bunch of functions and classes I have already made?

---- string.php ----

<?php
class String
{
    //contains the internal data
    var $data;

    // constructor
    function String($data) {
        $this->data = $data;
    }

    // creates a deep copy of the string object
    function copy() {
    }

    // adds another string object to this class
    function add($string) {
    }

    // returns the formated string
    function toString($format) {
    }
}
?>

---- testcase.php ----

<?php

require_once 'string.php';
require_once 'PHPUnit.php';

class StringTest extends PHPUnit_TestCase
{
    // contains the object handle of the string class
    var $abc;

    // constructor of the test suite
    function StringTest($name) {
       $this->PHPUnit_TestCase($name);
    }

    // called before the test functions will be executed
    // this function is defined in PHPUnit_TestCase and overwritten
    // here
    function setUp() {
        // create a new instance of String with the
        // string 'abc'
        $this->abc = new String("abc");
    }

    // called after the test functions are executed
    // this function is defined in PHPUnit_TestCase and overwritten
    // here
    function tearDown() {
        // delete your instance
        unset($this->abc);
    }

    // test the toString function
    function testToString() {
        $result = $this->abc->toString('contains %s');
        $expected = 'contains abc';
        $this->assertTrue($result == $expected);
    }

    // test the copy function
    function testCopy() {
      $abc2 = $this->abc->copy();
      $this->assertEquals($abc2, $this->abc);
    }

    // test the add function
    function testAdd() {
        $abc2 = new String('123');
        $this->abc->add($abc2);
        $result = $this->abc->toString("%s");
        $expected = "abc123";
        $this->assertTrue($result == $expected);
    }
  }
?>

---- stringtest.php ----

<?php

require_once 'testcase.php';
require_once 'PHPUnit.php';

$suite  = new PHPUnit_TestSuite("StringTest");
$result = PHPUnit::run($suite);

echo $result -> toString();
?>

TestCase stringtest->testtostring() failed: expected true, actual false
TestCase stringtest->testcopy() failed: expected , actual Object
TestCase stringtest->testadd() failed: expected true, actual false

---- string.php ----

<?php
class String
{
    //contains the internal data
    var $data;

    // constructor
    function String($data) {
        $this->data = $data;
    }

    // creates a deep copy of the string object
    function copy() {
        $ret = new String($this->data);
        return $ret;
    }

    // adds another string object to this class
    function add($string) {
        $this->data = $this->data.$string->toString("%ss");
    }

    // returns the formated string
    function toString($format) {
        $ret = sprintf($format, $this->data);
        return $ret;
    }
}
?>

~>
php -f stringtest.php

TestCase stringtest->testtostring() passed
TestCase stringtest->testcopy() passed
TestCase stringtest->testadd() failed: expected true, actual false

<?php
$this->data = $this->data.$string->toString("%s");
?> 

~>
php -f stringtest.php

TestCase stringtest->testtostring() passed
TestCase stringtest->testcopy() passed
TestCase stringtest->testadd() passed

// you will have to write your own class here that collects the tests
$collector = new Unit_Test_Collector();
$suite = $collector->getSuite();

//$config is an array of phpunit config options

PHPUnit_TextUI_TestRunner::run($suite, $config);