Final Challenge — Solution

Task: Add a findBySlug(string $slug): array method to the Post class,
used to support pretty URLs like post.php?slug=my-first-post instead of
an ID. It should throw PostNotFoundException if no matching post
exists, exactly like find(). Then write post.php, which reads
$_GET['slug'], calls the new method inside a try/catch, and either
displays the post (escaped with htmlspecialchars) or a friendly "Post
not found" message.

---- Post.php (findBySlug method added to the existing class) ----

<?php
    // ... existing Post class methods (create, find, all, update, delete) ...

    public function findBySlug(string $slug): array {
        $stmt = $this->pdo->prepare("SELECT * FROM posts WHERE slug = :slug");
        $stmt->execute(['slug' => $slug]);
        $post = $stmt->fetch();

        if (!$post) {
            throw new PostNotFoundException("No post found with slug '$slug'");
        }
        return $post;
    }
?>

---- post.php ----

<?php
    require __DIR__ . '/db.php';
    require __DIR__ . '/Post.php';

    $post = new Post($pdo);
    $slug = $_GET['slug'] ?? '';

    try {
        $article = $post->findBySlug($slug);
    } catch (PostNotFoundException $e) {
        $article = null;
    }
?>
<?php if ($article): ?>
    <h1><?= htmlspecialchars($article['title']) ?></h1>
    <p><?= htmlspecialchars($article['body']) ?></p>
<?php else: ?>
    <p>Post not found.</p>
<?php endif; ?>


Expected output (for post.php?slug=my-first-post, a genuinely existing post):
My First Post
Hello, world!

Expected output (for post.php?slug=does-not-exist):
Post not found.

Notes:
- findBySlug() mirrors find() almost exactly - same prepared-statement
  pattern, same PostNotFoundException thrown on a missing row - the
  only real difference is the WHERE clause matching on "slug" instead
  of "id".
- post.php reads $_GET['slug'] with the ?? '' safe-default pattern from
  Fundamentals Chapter 8, so a completely missing slug parameter
  doesn't trigger an undefined-key warning before the try/catch even
  runs.
- Both the post's title and body are escaped with htmlspecialchars()
  before being echoed, exactly matching index.php's own rule from the
  chapter - since a post's stored content is, ultimately, still a form
  of user input being redisplayed.
- The try/catch pattern deliberately sets $article to null on a caught
  PostNotFoundException rather than letting the exception propagate
  uncaught, which lets the rest of the page render a friendly message
  instead of crashing with a fatal, unhandled exception.