I recently got this warning message on one of WordPress site I am admin at. This error occurred after updating the WP site to latest version.
Warning: htmlspecialchars(): charset `UTF-7' not supported, assuming utf-8 in /home/bookis/public_html/wp-includes/formatting.php on line 3058
htmlspecialchars() is a PHP String function which converts special characters to HTML entities.So once a special character encounters it converts it to a HTML entities.
- ‘&’ (ampersand) becomes ‘&’
WordPress default charset it utf-8. During updating WordPress or theme etc some times the charset changes to UTF-7.
You Can see the changed charset in WP admin Dashboard in Settings => Reading.
It will be something like this-
The solution of the problem is that change character encoding to UTF-8 above and Refresh the site. The error should be resolved.
Let me know if you have any questions.