REST API: Avoid undefined property warning when creating a post with a slug

[abhishekrijal]

Trac ticket

Trac ticket: https://core.trac.wordpress.org/ticket/XXXXX

Problem

WP_REST_Posts_Controller::create_item() passes $prepared_post->id (lowercase) to
wp_unique_post_slug():

$prepared_post->post_name = wp_unique_post_slug(
    $prepared_post->post_name,
    $prepared_post->id, // lowercase — never set on this object
    'publish',
    $prepared_post->post_type,
    $prepared_post->post_parent
);

$prepared_post is a fresh stdClass from prepare_item_for_database(), which only ever assigns
$prepared_post->ID (uppercase), and only on the update path. On create, neither id nor ID is
set, so $prepared_post->id is an undefined property. Every other reference to the id in this file
already uses ->ID.

• PHP < 8: silent E_NOTICE, coerced to null → 0 (the correct value for a new post).
• PHP 8.0+: E_WARNING "Undefined property: stdClass::$id" on every matching request.

This fires on POST /wp/v2/posts when the request includes a slug and a draft/pending status
(common from the block editor), and is noisy in error loggers such as Sentry.

Fix

Pass the post ID defensively — 0 for a brand-new post — matching the ! empty( $prepared_post->ID )
guard already used elsewhere in the controller:

! empty( $prepared_post->ID ) ? $prepared_post->ID : 0,

Testing

Adds test_create_draft_with_slug_does_not_emit_undefined_property_warning() to
tests/phpunit/tests/rest-api/rest-posts-controller.php, which creates a draft with an explicit slug
and asserts (a) a 201 with the expected unique slug and (b) no Undefined property: stdClass::$id
warning is emitted during dispatch.

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Props aveeshek.

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[github-actions]

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