composer require phpunit/phpunit

./vendor/bin/phpunit

<?xml version="1.0" encoding="utf-8" ?>
<phpunit colors="true">
    <testsuite name="Mes super tests">
        <directory>./tests</directory>
    </testsuite>
    <filter>
        <whitelist proceessUncoveredFromWhitelist="true">
            <directory suffic=".php">src/</directory>
        </whitelist>
    </filter>
</phpunit>

// On crée un mock
$observer = $this->getMockBuilder('MaSuperClass')
  ->setMethods(array('getLastPost'))
  ->getMock();

// On précise les attentes concernant la méthode getLastPost
$observer->expects($this->once())
  ->method('getLastPost')
  ->with($this->equalTo('blabla'));

// On déroule notre code normalement
$observer->get();

.\vendor\bin\phpunit --coverage-html coverage