reflections.php

<?php

class TestCase extends \PHPUnit\Framework\TestCase
{
    public function testExample()
    {
        $refClass = new \ReflectionClass(\App\MyClass::class);
        $refMethod = $refClass->getMethod('somePrivateMethod');
        $refMethod->setAccessible(true); // make private method accessible

        $this->assertEqual(
            'expected value',
            $refMethod->invoke(new \App\MyClass, 'parameter for somePrivateMethod()')
        );
    }
}