The definition of this function is as follows:
class_exists ( string $class_name [, bool $autoload = true ]): bool
The second parameter is whether to automatically call the autoload function
class_name class name. The matching of names is not partition case. Whether autoload calls __autoload by default.
Of course, it is now recommended to use the following function for automatic loading
spl_autoload_register(function ($class_name) { require_once $class_name. ' .php ' ; });
If you don’t want to call, give false as the second parameter