What does the error "Undefined class name 'db'" in PHP indicate?

The error "Undefined class name 'db'" in PHP indicates that the class 'db' is not defined or included in the current script. To solve this issue, you need to make sure that the class 'db' is properly defined or included before it is used in the script. This can be done by including the file where the 'db' class is defined or making sure the class is declared in the same script.

// Include the file where the 'db' class is defined
require_once 'db_class.php';

// Now you can use the 'db' class
$db = new db();