I'm given example here of how to extend the sales order view page. Like this, you can extend any module which one you want. Follow the Below Steps
Step 1) Enable the extension to specify the config.php file from the app/etc folder
<?php
return array (
'modules' =>
array (
'Learning_RewriteSales' => 1,
),
);
Step 2) Create Custom.php Class in Block
<?php
namespace Learning\RewriteSales\Block\Adminhtml\Order\View;
class Custom extends \Magento\Backend\Block\Template
{
}
Step 3) Create Info.php file to extend core Info.php
<?php
namespace Learning\RewriteSales\Block\Adminhtml\Order\View;
class Info extends \Magento\Sales\Block\Adminhtml\Order\View\Info
{
}
Step 4) Create di.xml file to specify which class you were extended ( Dependency Injection )
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
<preference for="Magento\Sales\Block\Adminhtml\Order\View\Info" type="Learning\RewriteSales\Block\Adminhtml\Order\View\Info"/>
</config>
Step 5) Create module.xml file to specify the setup version
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
<module name="Learning_RewriteSales" setup_version="2.0.0">
<sequence>
<module name="Magento_Sales"/>
</sequence>
</module>
</config>
Step 6) Create sales_order_view.xml file from Learning/RewriteSales/view/layout folder and write the below code
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="admin-2columns-left" xsi:noNamespaceSchemaLocation="../../../../../../../lib/internal/Magento/Framework/View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="order_info">
<action method="setTemplate">
<argument name="template" translate="true" xsi:type="string">order/view/info.phtml</argument>
</action>
</referenceBlock>
<referenceBlock name="order_info">
<block class="Learning\RewriteSales\Block\Adminhtml\Order\View\Custom" name="sales_order_view_custom" template="order/view/custom.phtml" />
</referenceBlock>
</body>
</page>
Step 7) Create the Custom.phtml and Info.phtml file from Learning/RewriteSales/view/layout and insert below code.
Custom.phtml
<h1>Hi, I am here!</h1>
Info.phtml
First Copy the code from Magento\Sales\view\adminhtml\templates\order\view\info.phtml file and past here after that add one more line
<?php echo $block->getChildHtml('sales_order_view_custom');?>
Step 8) Finally clean the cache and see your custom code Sales Order View Page.
It's working for me. If you have any queries let me know?
Best Answer
adminhtml phtml file can be override as steps below
STEP 1: Add page layout XML in custom module.
STEP 2: Put template file on below path
STEP 3: Depending on your cache settings you may need to clear your cache
php bin/magento cache:clearHope it will work for you :)