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First of all we will define the responsibilities of our CurrencyRendererHelper. Basically, it will have two methods just for demonstration purpose:

This function will receive the amount to render. It will take 2 decimal digits filling empty space with zeros and prefix 'USD'.

This function will do the same as usd() but prefix the output with 'EUR'. Just to make it a bit more complex, we will also wrap the result in span tags:

<span class="euro"></span> 

1. <span class="euro"></span>

Let's create the tests first:

<?php

//Import the helper to be tested.
//If the tested helper were using some other helper, like Html, 
//it should be impoorted in this line, and instantialized in startTest().
App::import('Helper', 'CurrencyRenderer');

class CurrencyRendererTest extends CakeTestCase {
    private $currencyRenderer = null;

    //Here we instantiate our helper, and all other helpers we need.
    public function startTest() {
        $this->currencyRenderer = new CurrencyRendererHelper();
    }

    //testing usd() function.
    public function testUsd() {
        $this->assertEqual('USD 5.30', $this->currencyRenderer->usd(5.30));
	    //We should always have 2 decimal digits.
        $this->assertEqual('USD 1.00', $this->currencyRenderer->usd(1));
        $this->assertEqual('USD 2.05', $this->currencyRenderer->usd(2.05));
        //Testing the thousands separator
        $this->assertEqual('USD 12,000.70', $this->currencyRenderer->usd(12000.70));
    }
}

1. <?php
2. //Import the helper to be tested.
3. //If the tested helper were using some other helper, like Html,
4. //it should be impoorted in this line, and instantialized in startTest().
5. App::import('Helper', 'CurrencyRenderer');
6. class CurrencyRendererTest extends CakeTestCase {
7. private $currencyRenderer = null;
8. //Here we instantiate our helper, and all other helpers we need.
9. public function startTest() {
10. $this->currencyRenderer = new CurrencyRendererHelper();
11. }
12. //testing usd() function.
13. public function testUsd() {
14. $this->assertEqual('USD 5.30', $this->currencyRenderer->usd(5.30));
15. //We should always have 2 decimal digits.
16. $this->assertEqual('USD 1.00', $this->currencyRenderer->usd(1));
17. $this->assertEqual('USD 2.05', $this->currencyRenderer->usd(2.05));
18. //Testing the thousands separator
19. $this->assertEqual('USD 12,000.70', $this->currencyRenderer->usd(12000.70));
20. }
21. }

Here, we call usd() with different parameters and tell the test suite to check if the returned values are equal to what is expected.

Executing the test now will result in errors (because currencyRendererHelper doesn't even exist yet) showing that we have 3 fails.

Once we know what our method should do, we can write the method itself:

<?php
class CurrencyRendererHelper extends AppHelper {
    public function usd($amount) {
        return 'USD ' . number_format($amount, 2, '.', ',');
    }
}

1. <?php
2. class CurrencyRendererHelper extends AppHelper {
3. public function usd($amount) {
4. return 'USD ' . number_format($amount, 2, '.', ',');
5. }
6. }

Here we set the decimal places to 2, decimal separator to dot, thousands separator to comma, and prefix the formatted number with 'USD' string.

Save this in app/views/helpers/currency_renderer.php and execute the test. You should see a green bar and messaging indicating 4 passes.