Personal Catalogue: PHP & MySQL — Chapter 5, Exercise 2
====================================================

TASK
Add the pre-checked checkboxes and the sync logic (including the else
branch) to edit_item.php. Edit an existing book to remove one of its
two tags and add a third, then confirm item_tags reflects exactly the
new set — the removed tag's row gone, the third tag's row added, and
the untouched tag's row unchanged.

SOLUTION

Starting from "The Hobbit" (id, say, 4) with two tags attached —
Fantasy and Classic Fiction, per Exercise 1 — open edit_item.php?id=4.
The Fantasy and Classic Fiction checkboxes should already be checked
on page load (proof $itemTagIds and the in_array() check are wired
correctly).

Uncheck "Classic Fiction," leave "Fantasy" checked, and type a third,
new tag into the "New tag(s)" field:

    New tag(s): Adventure

Submit the form. The POST branch runs sync_item_tags($pdo, 4, $tagIds)
where $tagIds now contains [Fantasy's id, Adventure's newly-created id]
— Classic Fiction's id is genuinely absent from the submitted array,
since its checkbox was unchecked.

Confirm at the mysql> prompt:

    SELECT tags.name
    FROM tags
    JOIN item_tags ON item_tags.tag_id = tags.id
    WHERE item_tags.item_id = 4
    ORDER BY tags.name;

Expected output:

    +------------+
    | name       |
    +------------+
    | Adventure  |
    | Fantasy    |
    +------------+

Classic Fiction is gone (removed via sync_item_tags()'s own $toRemove
diff), Fantasy is still present and untouched (it was never in
$toRemove or $toAdd, so no DELETE or INSERT touched its row at all),
and Adventure is newly present (added via $toAdd after
find_or_create_tag() created it).

WHY THIS WORKS AS AN ANSWER
----------------------------
It exercises all three real outcomes sync_item_tags() is meant to
produce in one pass — a genuine removal, a genuine addition via a
brand-new tag, and an untouched tag left alone — and verifies the
final state directly against the database rather than only trusting
the form's own checkbox state after reload.