Magento – Where does the validation for onepage billing.phtml take place

To add form validation, you'll have to create the file to work on, and hook it to the Magento layout.

Add JavaScript to Sales Order Create

To follow best practices, you'll have to create your own module if you don't have already. I'll start from bottom up, to evidentiate the core need:

Create a layout file

app/design/adminhtml/default/default/layout/mycustom/adminthml.xml

<?xml version="1.0"?>
<layout>
    <adminhtml_sales_order_create_index>
        <reference name="head">
            <action method="addJs">
                <script>mycustom/mycustom.js</script>
            </action>
        </reference>
    </adminhtml_sales_order_create_index>
</layout>

You can skip the mycustom folder if you'd like and name your js file as you want - but remember to change accordingly also in the next steps.

Now you need to reference the xml layout file you just created.

Create your module

Create app\code\local\Mycustom\Adminhtml\etc\config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Mycustom_Adminhtml>
          <version>0.0.1</version>
        </Mycustom_Adminhtml>
    </modules>
    <adminhtml>
        <layout>
            <updates>
                <mycustom>
                    <!-- this will look for the XML file in the app/design/adminhtml/default/default/layout folder -->
                    <file>mycustom/adminhtml.xml</file>
                </mycustom>
            </updates>
        </layout>
    </adminhtml>
</config>

Again, you can also skip the folder and name the xml file as you want.

Now you need to reference your module. Create \app\etc\modules\Mycustom_Adminhtml.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Mycustom_Adminhtml>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Mage_Adminhtml/>
                <Mage_Sales />
            </depends>
        </Mycustom_Adminhtml>
    </modules>
</config>

Now you can create your Javascript file \js\mycustom\mycustom.js and put your validation code in it. For JS form validations you can check this answers: Custom form Validation - Magento

What to be aware of/check:

• Look into Configuration > Advanced > Advanced to see if your new module is listed and Enabled
• Log files in /var/log folder
• Typing errors
• Root of mentioned paths is /web/ of your Magento installation folder

More detailed version here: Add JS to Magento Sales Order

You need to implement the switch for this validation on client-side.

What you are trying above is, to check the country during template generation. It could work only once (on load) when the customer selected India before but it won't because it's syntactically wrong in any way: <?php if($country!=India)?> as $country is the output for the dropdown and it's syntactically wrong in PHP. 'India' needs to be put in quotes.

What I recommend:

1. Remove the if/else from the phtml file
2. Add a JavaScript event that fires when the checkbox is changed.
3. On change of the country dropdown, add/remove the validation class (<?php echo $this->helper('customer/address')->getAttributeValidationClass('city') ?>) from the input field class attribute.