Code Quality: Resolve variable.undefined errors in wp-mail.php and wp-trackback.php - #13880

Trac ticket: https://core.trac.wordpress.org/ticket/65817
Follow-up to #13810 (review)

Resolves three variable.undefined baseline entries covering six error occurrences in wp-mail.php and wp-trackback.php. The wp-activate.php and wp-signup.php changes are handled separately in PR #13810.

Changes

1. src/wp-mail.php: Initialize $subject = ''; alongside the other per-message variables ($bodysignal, $boundary, $charset, etc.) before the foreach ( $message as $line ) loop. Without this, $subject is only assigned when a line matches /Subject: /i, so PHPStan correctly flags it as potentially undefined when the loop completes without a match.

2. src/wp-trackback.php: Add global $wpdb, $posts; with @global docblocks in file scope. $wpdb is used four times (two prepare calls, one get_results, one insert_id) and $posts once ($posts[0]->ID), all reachable after require wp-load.php.

3. tests/phpstan/baselines/variable.undefined.neon: Remove the three resolved suppression entries (6 errors total).

Background

• The $subject variable in wp-mail.php has been conditionally assigned inside the header-parsing loop since the file's earliest form. The surrounding variables ($bodysignal, $boundary, $charset, $content, $content_type, $content_transfer_encoding, $post_author, $author_found, $post_date, $post_date_gmt) were all given explicit initializations in r42343 (2017-11-30, Gary Pendergast), but $subject was missed.

• The $wpdb and $posts usages in wp-trackback.php date to the earliest revisions of the file. Variable names were cleaned up in r53719 (2022-07-19, Sergey Biryukov), but no global declaration was added since the file operates at the global scope via require.

Testing instructions

1. On trunk, npm run typecheck:php reports [OK] No errors, because all three occurrences are baselined.
2. Remove the three entries from tests/phpstan/baselines/variable.undefined.neon without applying the source changes, then run again. PHPStan reports:
   • Variable $subject might not be defined. in src/wp-mail.php
   • Variable $wpdb might not be defined. (×4) in src/wp-trackback.php
   • Variable $posts might not be defined. in src/wp-trackback.php
3. With this branch applied, npm run typecheck:php reports [OK] No errors with all three baseline entries removed and no new errors elsewhere.
4. composer lint is clean on both changed files.
5. The baseline was regenerated with composer phpstan:baselines -- --identifier=variable.undefined, not edited by hand.

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity IDE
Model(s): Claude Opus 4
Used for: Locating the variable.undefined occurrences via the PHPStan baseline, git blame/git log -S archaeology to trace each variable's origin, and drafting this description. The source changes, baseline regeneration, and verification against PHPStan and PHPCS were reviewed and confirmed by me in a local development environment.