Fix W3 Total Cache DbCache fatal in batch_insert

[ilicfilip]

Summary

Fixes a fatal error reported by a user via WP.org support topic #4979 running this plugin alongside W3 Total Cache with Database Cache enabled:

PHP Fatal error: Uncaught TypeError: mysqli_num_fields(): Argument #1 ($result) must be of type mysqli_result, true given
in /wp-includes/class-wpdb.php:3876
…
#3 /wp-content/plugins/w3-total-cache/DbCache_WpdbInjection_QueryCaching.php(230): wpdb->__get('col_info')
#4 /wp-content/plugins/w3-total-cache/DbCache_WpdbNew.php(216): W3TC\DbCache_WpdbInjection_QueryCaching->query()
#5 /wp-content/plugins/aaa-option-optimizer/src/class-database.php(199): W3TC\DbCache_WpdbNew->query()

The crash fires on shutdown when Database::batch_insert() issues BEGIN.

Root cause

W3TC's DbCache layer wraps $wpdb and, after each cacheable query, reads $wpdb->col_info to stash for the cache. That triggers wpdb::load_col_info() → mysqli_num_fields($this->result). For non-SELECT queries the mysqli_query result is the boolean true — hence the type error.

W3TC has an explicit bypass that disables caching for transaction control and write statements via this regex (DbCache_WpdbInjection_QueryCaching.php):

~^\s*start transaction\b~is
~^\s*insert\b|^\s*delete\b|^\s*update\b|^\s*replace\b|^\s*commit\b|^\s*truncate\b|^\s*drop\b|^\s*create\b~is

Note the gap: START TRANSACTION and COMMIT are matched, but BEGIN (a MySQL alias for START TRANSACTION) is not. Our INSERT queries and the closing COMMIT already short-circuit caching, but the opening BEGIN slips through — and that's the query that fatals.

Fix

Use START TRANSACTION instead of BEGIN in Database::batch_insert(). The two are semantically identical in MySQL, but START TRANSACTION matches W3TC's bypass regex, so the col_info path is never entered.

Testing

Reproduced locally against a Valet site with W3TC 2.9.4 (DbCache enabled, file engine):

• Before the fix: wp eval 'Database::batch_insert([...])' produces the identical stack trace to the user's report (same files, same line numbers, same mysqli_num_fields(true) fatal).
• After the fix: batch_insert() completes cleanly; rows land in wp_option_optimizer_tracked as expected.

Notes

• Reported via https://wordpress.org/support/topic/fatal-error-4979/ (plugin version 1.6.1).
• Version bump and changelog entry are intentionally not part of this PR — will be done before the next release per Filip.
• This is a one-line behavior-preserving change; no functional impact when W3TC is not in use.

🤖 Generated with Claude Code

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