It's relatively easy to test Entities, simply because they're plain old PHP classes with actions derived from the Domain concept they represent. The focus of the test should be the invariants that the Entity protects, because the behavior on the Entities will likely be modeled around those invariants.

For example, and for the sake of simplicity, suppose a Domain Model for a blog is needed. A possible one could be this:

class Post
{
    private $title;
    private $content;
    private $status;
    private $createdAt;
    private $publishedAt;

    public function __construct($aContent, $title)
    {
        $this->setContent($aContent);
        $this->setTitle($title);

        $this->unpublish();
        $this->createdAt(new DateTimeImmutable());
    }

    private function setContent($aContent)
    {
        $this->assertNotEmpty($aContent);

        $this->content = $aContent;
    }

    private function setTitle($aPostTitle)
    {
        $this->assertNotEmpty...