require_once(dirname(__FILE__).'/classes/mymodule_class.php');

if (!defined('_PS_VERSION_')) {
    exit;
}

class mymodule_class{

    protected static $module = false;

    public function test(){
        return 'test';
    }
 }

print_r(mymodule_class::test)

$mymodule_class = new mymodule_class();
print_r (mymodule_class->test());