Dependency injection through constructor or function parameter

Hi developers,

I was wondering what the difference is between injecting a (service) class through the constructor of a class and injecting a class as a function parameter. I've tested both approaches, and they both work exactly the same. Why should you inject it through the constructor? What is the purpose of this over injecting it as a function parameter, since they do the same?

I've got an example below. I did not bind anything to the service container manually.

PostService.php

<?php

namespace App\Services;

use App\Models\Post;

class PostService
{
    /**
     * Convert the post to PDF.
     *
     * @param Post $post
     * @return string
     */
    public function convertToPdf(Post $post): string
    {
        return 'PDF';
    }
}

PostController.php

<?php

namespace App\Http\Controllers;

use App\Models\Post;
use App\Services\PostService;

class PostController extends Controller
{
    private PostService $postService;

    public function __construct(PostService $postService)
    {
        $this->postService = $postService;
    }

    /**
     * This is through dependency injection in the controller's parameter.
     *
     * @param Post $retrospective
     * @param PostService $postService
     * @return string
     */
    public function throughConstructor(Post $retrospective, PostService $postService): string
    {
        return $postService->convertToPdf($retrospective->id);
    }

    /**
     * This is through dependency injection through the constructor.
     *
     * @param Post $retrospective
     * @return string
     */
    public function throughFunctionParameter(Post $retrospective): string
    {
        return $this->postService->convertToPdf($retrospective->id);
    }
}