In Magento 1.9, based on a condition, change checkout/onepage layout

Question

I want to change a checkout/onepage layout based on a condition.

Example:

<?php
    $coupon = Mage::getSingleton('checkout/session')->getQuote()->getCouponCode();
    if(($coupon == "SE50") || ($coupon == "SE00")){
    }
?>

To showcheckout.xml

<action method="setTemplate"><template>page/2columns-right.phtml</template></action>

I mean this page:

if(empty($coupon))

<action method="setTemplate"><template>page/checkout-onepage.phtml</template></action>

A page like:

Note: checkout-onepage.phtml is a custom page.

Answer 1

You can use a helper function to set the template based on a condition. Follow the below code.

Change the action tag in your layout.xml to the below.

 <action method="setTemplate"><template helper="your_module/getLayout" /></action>

In your module's helper file Data.php add the below code.

public function getLayout()
{
    $coupon = Mage::getSingleton('checkout/session')->getQuote()->getCouponCode();
    if(($coupon == "SE50") || ($coupon == "SE00")){

        return 'page/2columns-right.phtml';
    }
    else
    {
        return 'page/checkout-onepage.phtml';
    }
}

Answer 2

Either change your checkout.xml (or better add this to your local.xml)

...
<action method="setTemplate">
    <template helper="module/helper/yourMethod" />
</action>
...

Add a custom module with a helper method like ...

public function yourMethod()
{
    $coupon = Mage::getSingleton('checkout/session')->getQuote()->getCouponCode();
    if ($coupon == 'SE50' || $coupon == 'SE00') {
        return 'page/2columns-right.phtml';
    } else {
        return 'page/2columns-left.phtml';
    }
}