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

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