The code referenced a class that does not exist.
WHAT IT MEANS
PHP could not find the class being used.
THE COMMON CAUSES
Dependencies not installed. For Composer projects, vendor is missing or incomplete. An incomplete file upload A missing PHP extension providing the class An autoloader not configured or not running A plugin expecting another that is not active
FOR COMPOSER PROJECTS
Run composer install. The vendor directory is frequently excluded from transfers and must be installed on the server.
If Composer fails on memory, run it with a raised limit.
FOR EXTENSION CLASSES
Classes such as Imagick, PDO or ZipArchive come from extensions. Enable them under cPanel > Select PHP Version.
AFTER A MIGRATION
Check that all files transferred. The vendor directory contains very many small files and frequently fails to transfer completely.
AFTER AN UPDATE
A component may expect a newer version of something else. Check whether related components need updating too.
THE ERROR NAMES THE CLASS
Searching for it identifies which library or extension it belongs to.
