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Updated: 11 October 2026 · Applies to: WordPress 7.1

This error comes from PHP code that uses REQUEST_URI without the $_SERVER array, usually in an old theme or plugin. In PHP 8 it is no longer a warning but a fatal error ("Undefined constant"), which can make the site go blank.

Fix

  1. Find the file and line: enable WordPress debugging (define( 'WP_DEBUG', true ); and define( 'WP_DEBUG_LOG', true ); in wp-config.php) and read wp-content/debug.log.
  2. Update the theme or plugin named there. If no update exists, the code must use $_SERVER['REQUEST_URI'] instead of REQUEST_URI.
  3. Turn WP_DEBUG off again.

Tip: Temporarily switching the site to an older PHP version can buy time, but replacing the outdated code is the real fix.

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