You should include the class in your plugin if possible. I have had scenarios where a class was part of the theme, but also needed in a plugin where you couldn' t assume the class was. PHP Fatal error: Class ' WP_ Widget' not found in / home4/ tandem/ public_ html/ tandemme/ wp- content. Tandem Middle East is a licensed Training provider headquartered. suddenly getting this error Fatal error: Class ' WP_ Widget_ Tag_ Cloud' not found in class- wp- widget- factory. php on line 106 No changes at all. Deactivated all plugins. Fatal error: Class ' Proprietaires_ Widget' not found in / opt/ lampp/ htdocs/ tests/ wordpress/ wp- includes/ class- wp- widget- factory. I do not see where I could have been wrong.