Need help making checkout comments required

[igofresh]

My site is www.igofresh.com. Linux hosting and ZC 1.3.9g.
I am trying to make the comments field in checkout required before the customer can move on from step 1.
In checkout_shipping.php?
Many thanks in advance

[Ajeh]

You could customize the file:
/includes/modules/pages/checkout_payment/header_php.php

//  $_SESSION['comments'] = '';
$comments = $_SESSION['comments'];
if (trim($comments) == '') {
    $messageStack->add_session('checkout_shipping', 'Complete the comments', 'error');
    zen_redirect(zen_href_link(FILENAME_CHECKOUT_SHIPPING, '', 'SSL'));
}

[igofresh]

Thanks for the reply.
That didn't work for me.
I may have problems because I have changed the comments from a simple comment box in checkout_shipping.php to a drop down with 3 choices. I noticed in payment_shipping.php the comment box is populated with the "select one" message from the drop down. even if nothing is selected in the previous page's comment box.

[igofresh]

Now it works.
I removed the text first option from the drop down so rather than saying '--chose an option--' it is just blank. This allows Ajeh's solution to work as intended.
The only potential problem I foresee in that in step 2 of checkout the customer may decide to change their mind and type to edit the delivery option they chose in the previous step.
Can the comment box or part or all of its contents be protected in page 2?

Just for those who go down this post in the future,
I added a drop down to replace the text input box in checkout comments. In my case, I needed to offer the customer 3 delivery time slots to choose from. I found that solution here:
http://www.zen-cart.com/forum/showthread.php?t=107227
The reason for using comments is that it loads onto the invoice and other order docs.
The last piece of the puzzle was to make certain the customer selected a delivery option and did not skip this step.
That is the solution posted here by Ajeh.
All together it works like a charm.

[igofresh]

I have now added some virtual products and get a redirect loop error message rather than going to step 1 of checkout.
Before implementing the comments field requirement as described above check out

http://www.zen-cart.com/forum/showthread.php?t=73695

I like this option better because it makes comments required by modifying checkout_confirmation/header_php.php not checkout_payment/header_php.php
If you have only virtual product(s) in the cart checkout_payment/header_php.php will try to reload
checkout_shipping which is skipped for virtual products!
Theres the loop!

[Ajeh]

Thanks for the update on what worked for you on making comments required ...