Invoice.php

<?php

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
#[ORM\Table(name: "invoice")]
class Invoice
{
    #[ORM\Id]
    #[ORM\GeneratedValue]
    #[ORM\Column(type: "integer")]
    private int $id;

    #[ORM\Column(type: "string", length: 255)]
    private string $invoiceNumber;

    #[ORM\Column(type: "decimal", precision: 10, scale: 2)]
    private string $amount;

    #[ORM\Column(type: "datetime")]
    private \DateTimeInterface $createdAt;

    // Constructor
    public function __construct(string $invoiceNumber, string $amount)
    {
        $this->invoiceNumber = $invoiceNumber;
        $this->amount = $amount;
        $this->createdAt = new \DateTimeImmutable();
    }

    // ...
}

InvoiceController

<?php
// ...
use App\Entity\Invoice;
use Symfony\Component\Serializer\SerializerInterface;

#[Route('/invoices')]
class InvoiceController extends AbstractController
{
    #[Route('/{id}', name: 'invoice_show', methods: ['GET'])]
    public function show(Invoice $invoice, SerializerInterface $serializer): JsonResponse
    {
        $jsonData = $serializer->serialize($invoice, 'json');

        return new JsonResponse($jsonData, 200, [], true);
    }
}