Why won't Memcached start?

Question

PHP

Why doesn't Memcached start in classes?
For example if

$m = new Memcached();
        $m->addServer('localhost', 11211);

in a regular file enter works, but if it's in a class

class aaa
{

    public function __construct($id)
    {
        $m = new Memcached();
        $m->addServer('localhost', 11211);

gives an error message

Fatal error: Uncaught Error: Class 'modules\Memcached' not found in /var/www/html/modules/aaa.php:4

Answer 1

gives an error message

Of course, I will not read the error, and show it too