Source code: The Basic Workflow of Writing Unit Tests in PHPUnit

Calculator.php

<?php

class Calculator
{
    private $result;

    function __construct($initial = 0)
    {
        $this->result = $initial;
    }

    public function getResult()
    {
        return $this->result;
    }

    public function add($number)
    {
        $this->result += $number;

        return $this;
    }

    public function sub($number)
    {
        $this->result -= $number;

        return $this;
    }
}

CalculatorTest.php

<?php

class CalculatorTest extends PHPUnit_Framework_TestCase
{
    /** @test */
    public function it_can_start_with_an_initial_number()
    {
        $calculator = new Calculator(8);

        $this->assertEquals(8, $calculator->getResult());
    }

    /** @test */
    public function it_starts_with_zero_if_nothing_is_specified()
    {
        $calculator = new Calculator;

        $this->assertEquals(0, $calculator->getResult());
    }

    /** @test */
    public function it_adds_numbers()
    {
        $calculator = new Calculator(2);

        $result = $calculator->add(3)->getResult();

        $this->assertEquals(5, $result);
    }

    /** @test */
    public function it_subtracts_numbers()
    {
        $calculator = new Calculator(10);

        $result = $calculator->sub(2)->getResult();

        $this->assertEquals(8, $result);
    }

    /** @test */
    public function it_can_mix_operations()
    {
        $calculator = new Calculator(10);

        $result = $calculator->add(2)->sub(5)->getResult();

        $this->assertEquals(7, $result);
    }

}