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registration.php

 <?php
 MagentoFrameworkComponentComponentRegistrar::register(
 MagentoFrameworkComponentComponentRegistrar::MODULE,
 'Hafiz_Sms',
 __DIR__
 );

etc/events.xml

<?xml version="1.0"?>
<configxmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<eventname="sales_order_place_after">
<observername="place_order_after" instance="HafizSmsObserverOrderplaceafter" />
</event>
</config>

etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
<module name="Hafiz_Sms" setup_version="2.1.1"></module>
</config>

Observer/Orderplaceafter.php

<?php
namespace HafizSmsObserver;
use MagentoFrameworkEventObserverInterface;
use PsrLogLoggerInterface;
class Orderplaceafter implements ObserverInterface

protected $logger;
public function __construct(LoggerInterface$logger) 
$this->logger = $logger;

public function execute(MagentoFrameworkEventObserver $observer)
 $order = $observer->getEvent()->getOrder();
 echo $orderId = $order->getId();
// $comment = $this->getRequest()->getParam('comment');
 print_r("Catched event succssfully !"); exit;

But it's not working. How to debug in this event?. Kindly suggest me where i am wrong?

The event will be defined at etc/events.xml Nor etc/event.xml.Seems file name is wrong.

events.xmlcode will be like.Also, the Syntax of declare event is wrong. Please used below code:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
 <event name="sales_order_place_after">
 <observer name="place_order_after" instance="HafizSmsObserverOrderplaceafter" />
 </event>
</config>

Follow this article for how to implement event at module https://devdocs.magento.com/guides/v2.2/extension-dev-guide/events-and-observers.html.

Forgetting purpose, don't use print_r, die and exit. Use Magento loggger for print for debugging Magento 2: Replacement for Mage::log method?

Yes, it is an interesting question. Many people may know about sales_order_place_after event.

If you use this event, you can't use print_r() and exit() function.

Otherwise, kindly use checkout_onepage_controller_success_action event. It will a helpful lot

You declared event name and observer name as observername and eventname.

<?xml version="1.0"?>
<configxmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
<event name="sales_order_place_after">
<observer name="place_order_after" instance="HafizSmsObserverOrderplaceafter" />
</event>
</config>