Пример сущности с необязательными полями

Act.php

<?php

namespace App;

class Act
{
    private int $id;
    private string $title;
    private ?\DateTimeInterface $signedAt;

    private function __construct(
        int $id,
        string $title,
        ?\DateTimeInterface $signedAt
    ) {
        $this->id = $id;
        $this->title = $title;
        $this->signedAt = $signedAt;
    }

    public static function fromUserForm(string $title): self
    {
        $record = Db::insert('acts', [
            'title' => $title,
            'signed_at' => null,
        ]);

        return new self(
            $record->id,
            $title,
            null
        );
    }

    public static function fromDb(int $id): self
    {
        $record = Db::get('acts', [
            'id' => $id,
        ]);

        return new self(
            $record->id,
            $record->title,
            $record->signed_at
        );
    }

    public function sign(\DateTimeInterface $dateTime): void
    {
        $this->signedAt = $dateTime;

        Db::update(
            'acts',
            [
                'id' => $this->id,
            ],
            [
                'signed_at' => $this->signedAt->format('Y-m-d'),
            ]
        );
    }

    public function getTitle(): string
    {
        return $this->title;
    }

    public function getSignedAt(): ?\DateTimeInterface
    {
        return $this->signedAt;
    }
}