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if you need use variable  instead number the store the value in database like  

//save the value in Configuration table in databse
Configuration::updateValue('YOUR_NAME_ORDER', 21);

//get the value from Configuration table
Configuration::get('YOUR_NAME_ORDER');

//Then after compare value with your order status

public function hookActionOrderStatusPostUpdate($params)
{
	$newOrderStatus = $params['newOrderStatus'];
	if($newOrderStatus->id == Configuration::get('YOUR_NAME_ORDER')) {
      //your code here
    }
}

Thanks