array(0) { } Warning: Cannot modify header information ERROR! Help!

array(0) { }
Warning: Cannot modify header information - headers already sent by (output started at /home/led2014/public_html/checkout_failure.php:1) in /home/led2014/public_html/checkout_failure.php on line 17

This has just started happening today, despite not having made any changes to checkout_failure.php. Site is ledwarehouseuk.com.
Hope you can help. FTP details are in my other post which also keenly awaits your reply.
Thank you.
NJ

the following code was at line 1 of the failure page to write the session contents:

<?php @session_start(); var_dump($_SESSION); ?>

i commented the var_dump to stop the error.

Perhaps error reporting had been turned off before and had gotten turned on by the host.

The site is on a VPS, which we manage and we haven't touched it. :(

All transactions on the site have stopped working. My client sent me the following screenshot, which I hope might help you track down the issue. This was with a Visa Debit.

I tried my own personal Mastercard and it didn't produce this massive error script (as per the attached screenshot), but did take me to the checkout_failure.php page. My card is good so there is definitely something wrong.

Hope you can help. Thank you.
NJ

The content in the head is the var_dump that I commented.

I am forwarding this to Ray to have him look into.