Using a trait to share test code for multiple implementations

gistfile1.php

trait RepositoryContract
{
     public function testAdd() { ... }
     public function testRemove() { ... }
}

class MongoDBRepositoryTest {
    use RepositoryContract;
    public function setUp()
    {
         // create a connection, clean up the state...
         $this->repository = new MongoDBRepository(/* ... */);
    }
}

class InMemoryRepositoryTest {
    use RepositoryContract;
    public function setUp()
    {
         $this->repository = new InMemoryRepository();
    }
}