dummy-repo.php

<?php
class FooEntity
{
    public string $entityId;
    public DateTime $date;

    public function __construct(string $entityId, DateTime $date)
    {
        $this->entityId = $entityId;
        $this->date = $date;
    }
}

interface FooRepository
{
    public function doSomething(FooEntity $entity): void;
    public function get(string $fooId) : FooEntity;
}

class FooClient
{
    private FooRepository $repository;

    public function __construct(FooRepository $repository)
    {
        $this->repository = $repository;
    }
    
    public function heyJoe(): void
    {
        $entity = new FooEntity('entityId-123', DateTime::createFromFormat('d/m/Y', '10/04/1987'));
        $this->repository->doSomething($entity);
    }
}

class FooClientTest
{
    public function test_repository_calls_correctly() : void
    {
        
        $repository = new class implements FooRepository
        {
            /** @var array<FooEntity>  */
            private array $storage = [];
            
            public function doSomething(FooEntity $entity): void
            {
                $this->storage[$entity->entityId] = $entity;    
            }
            
            public function get(string $fooId) : FooEntity
            {
                return $this->storage[$fooId];
            }
        };
        
        $client = new FooClient($repository);
        $client->heyJoe();
        
        self::assertEquals('some value', $repository->get('someId'));
    }
}