Plesk Wordpress website with PHP 8 is inaccessible: There has been a critical error on this website

Symptoms

• WordPress website displays the following error in a browser:

  CONFIG_TEXT: There has been a critical error on this website.
  Learn more about debugging in WordPress.

• In Plesk > Domains > example.com > Logs the error message like the following can be found:

  CONFIG_TEXT: AH01071: Got error 'PHP message: PHP Parse error: syntax error, unexpected token "\", expecting end of file in /var/www/vhosts/example.com/wp-content/themes/eleven40/functions.php on line 194'

  ---

  CONFIG_TEXT: AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to undefined function create_function() in /var/www/vhosts/example.com/wp-content/plugins/arscode-ninja-widget/arscode-ninja-widget.php:140\nStack trace:\n#0 /var/www/vhosts/example.com/wp-settings.php(388): include_once()\n#1 /var/www/vhosts/example.com/wp-config.php(80): require_once('...')\n#2 /var/www/vhosts/example.com/wp-load.php(37): require_once('...')\n#3 /var/www/vhosts/example.com/wp-blog-header.php(13): require_once('...')\n#4 /var/www/vhosts/example.com/index.php(17): require('...')\n#5 {main}\n thrown in /var/www/vhosts/example.com/wp-content/plugins/arscode-ninja-widget/arscode-ninja-widget.php on line 140'

  ---

  CONFIG_TEXT: AH01071: Got error 'PHP message: PHP Warning: Constant WP_DEBUG already defined in /var/www/vhosts/example.com/public_html/wp-config.php on line 80PHP message: PHP Fatal error: Uncaught Error: Attempt to assign property "plugins" on bool in /var/www/vhosts/example.com/public_html/wp-content/plugins/simply-gallery-block/freemius/start.php:74\n...

  ---

  CONFIG_TEXT: PHP message: PHP Warning: Undefined variable $madara_ajax_search in /var/www/vhosts/example.com/httpdocs/wp-content/themes/madara/madara-core/manga-search.php on line 48

• PHP version 8.0 is selected in Domains > example.com > PHP Settings.

Cause

Theme/plugin from the error message is incompatible with PHP 8.

Resolution

Apply one of possible solutions: