PHP 8.3 Migration for WordPress: A Checklist That Worked on 47 Sites

PHP 8.3 is the deadline most WordPress hosts moved to in 2025. Our migration of 47 client sites broke things on six of them — about one in eight. Here is the checklist that handled the other 41 cleanly, and the patterns of the six that did not.

The pre-migration checklist

  1. WordPress core on the latest minor. 6.5+ is fine with 8.3. Older cores have unresolved deprecation warnings that are now fatal.
  2. All active plugins on their latest version. Plugin compatibility regressions are 80 percent of the breakage we saw.
  3. Theme PHP files audited for deprecated functions. create_function(), the old each() pattern, dynamic class properties without declaration.
  4. Test a staging clone first. Always. We never migrated production without a staging pass.

What actually broke

Across the six sites that hit issues, the patterns were:

  • Dynamic properties on undeclared classes. 8.2 deprecated this; 8.3 makes it a fatal in some cases. Almost always in an abandoned plugin we then replaced.
  • Calling strtolower() on null. Strict type changes mean what used to be a soft warning is now a fatal. Audit any code that handles user input or plugin metadata.
  • Removed utf8_encode and utf8_decode. These were deprecated then removed. Old import plugins were the worst offenders.
  • SOAP and ext-imap quirks. Two sites used ancient SOAP integrations that needed a workaround.

The migration step-by-step

  1. Clone production to staging.
  2. Switch staging to PHP 8.3 in your host’s control panel or via WP-CLI’s host-specific command.
  3. Visit every public template and the entire admin. Note any white screen or error.
  4. Tail wp-content/debug.log for one hour of normal use. Almost everything that will fail in production fails here first.
  5. Run WooCommerce checkout end-to-end on test cards if it is a store.
  6. Cut over production. Keep the 8.2 environment available for 48 hours as a rollback.

The single most useful thing

Enable WP_DEBUG_LOG with display_errors = Off on staging for the migration window. Every deprecation that will bite you logs there first. It is the cheapest insurance you can buy.

8.3 is faster and the migration is usually clean. The pattern across the sites that broke was always “old plugin nobody touches” — the migration was a useful prompt to retire those.