Generics in PHP

Example 1: A simple generic Collection class

Define the generic class (note the use of @template TItem):

<?php

/**
 * @template TItem
 */
final readonly class Collection
{
    /**
     * @param array<int, TItem> $items
     */
    public function __construct(private array $items) {}

    /**
     * @return array<int, TItem>
     */
    public function all(): array
    {
        return $this->items;
    }
}

Use the class:

<?php

$names = new Collection(['Bob' ,'Jim', 'Mike', 'Alice']);

$names = $names->all();

foreach($names as $index => $name) {
    echo strtolower($name) . PHP_EOL;
}

Example 2

<?php

declare(strict_types=1);

namespace App\Repository;

use App\Entity\User;
use App\Exception\EntityNotFoundException;
use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;

/**
 * @template TEntity of object
 * @extends ServiceEntityRepository<TEntity>
 */
class BaseRepository extends ServiceEntityRepository
{

    /**
     * @return TEntity
     * @throws EntityNotFoundException
     */
    public function findOrFail(int $id)
    {
        $user = $this->find($id);

        if (!$user) {
            throw new EntityNotFoundException(User::class, $id);
        }

        return $user;
    }

    /**
     * @return TEntity|null
     */
    public function findFirst()
    {
        return $this->createQueryBuilder('o')
            ->setMaxResults(1)
            ->getQuery()
            ->getOneOrNullResult();
    }
}