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CacheWarmer.php

<?php

namespace My\App;

class CacheWarmer
{
    private $cacheDirectory;

    public function __construct(string $cacheDirectory)
    {
        $this->cacheDirectory = $cacheDirectory;
    }

    public function warmUp()
    {
        if (!is_dir($this->cacheDirectory) &&!mkdir($this->cacheDirectory)) {
            throw new \RuntimeException('Could not create cache directory!');
        }

        // ...
    }
}

CacheWarmerTest.php

<?php

namespace My\App\Tests;

use My\App\CacheWarmer;
use org\bovigo\vfs\vfsStream;

class CacheWarmerTest extends \PHPUnit_Framework_TestCase
{
    private $root;

    public function setUp()
    {
        $this->root = vfsStream::setup();
    }

    /**
     * @test
     */
    public function canCreateCacheDirectoryOnWarmUp()
    {
        $cacheWarmer = new CacheWarmer($this->root->url() . '/cache');
        $cacheWarmer->warmUp();

        $this->assertTrue($this->root->hasChild('cache'));
    }
}

PdfCreator.php

<?php

namespace My\App;

class PdfCreator
{
    // ...

    public function execute(string $htmlFile, string $pdfFile)
    {
        $output = [];
        $returnValue = 0;

        exec(sprintf('/usr/bin/wkhtmltopdf %s %s', $htmlFile, $pdfFile), $output, $returnValue);

        if ($returnValue !== 0) {
            throw new \RuntimeException('Could not create PDF file!');
        }

        return $output;
    }

    // ...
}

PdfCreatorTest.php

<?php

namespace My\App\Tests;

use My\App\PdfCreator;

class PdfCreatorTest extends \PHPUnit_Framework_TestCase
{
    use \phpmock\phpunit\PHPMock;

    /**
     * @test
     * @expectedException \RuntimeException
     */
    public function executeReturnsAnExceptionOnFailure()
    {
        $exec = $this->getFunctionMock('My\App', 'exec');
        $exec->expects($this->once())->willReturnCallback(
            function ($command, &$output, &$returnValue) {
                $this->assertEquals('/usr/bin/wkhtmltopdf file.html file.pdf', $command);
                $output = ['failure'];
                $returnValue = 1;
            }
        );

        $pdfCreator = new PdfCreator;
        $pdfCreator->execute('file.html', 'file.pdf');
    }
}