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When using the Yoast SEO plugin in WordPress, adding certain non-Latin characters (e.g., Cyrillic) in the Gutenberg editor results in a fatal error due to a missing PHP function. This breaks the front-end rendering and schema generation.

To Reproduce

Step-by-step reproduction instructions

1. Use WordPress with the Yoast SEO plugin enabled.

2. Open the Gutenberg editor.

3. Add this text to the post content: За здоровье!

4. Save and publish the post.

5. Attempt to visit the front-end of the post.

Expected results

Yoast should gracefully handle multibyte content or verify that required PHP extensions (like mbstring) are enabled before calling functions like mb_strtoupper().

Actual results

A fatal error is thrown:

• Uncaught Error: Call to undefined function mb_strtoupper()
  in /var/www/xyz.com/wp-content/plugins/wordpress-seo/src/generators/schema/article.php on line 230

This prevents the post from loading properly on the frontend.

Screenshot

Technical info

• If relevant, which editor is affected:

  • Block Editor
  • Gutenberg Editor
  • Elementor Editor
  • Classic Editor
  • Other:

• Which browser is affected:

  • Chrome
  • Firefox
  • Safari
  • Other:

Used versions

• Device you are using: Laptop
• Operating system: Windows 10
• PHP version: 8.0
• WordPress version: 6.8.1
• WordPress Theme: Custom theme
• Yoast SEO version: 25.0
• Gutenberg plugin version: 20.7.0