My website is entirely blank, tips to diagnose? - debugging

Question

I have an ecommerce site built on OpenCart (1.5.0 i believe), which after inserting tracking code from Alexa.com and a block of code for redirecting to another site went entirely blank. Initially it worked, so I felt it was safe to save over my backups. But after refreshing the pages it was blank, and the 'view source' option in all my browsers (firefox, chrome, safari) revealed that there was no code reaching them. I then began to follow a series of debugging steps:
As my text editor was still open I undid all changes and reuploaded with no changes
I scanned the documents to be sure there were no issues in the text, with no discoveries
I ran it through W3C validation, with 2 warnings which are 1- no character encoding and 2- unable to determine parse mode.
contacted host for a server side restore though their earliest backup was a day after the problem began (it took 3 days of arguing to get them to initiate a restore)
In regards to these validation warnings, I am not sure what encoding should be used for OpenCart, ASCII or UTF-8 (which the validator resorted to) or what, in addition I am not sure if the template used with OpenCart would conflict with it if I were to declare encoding. In addition, I find it hard to believe that such a widely distributed product would have something this simple causing such a huge mistake because then all users would have this issue.
In regards to parse mode, the index page does declare parse mode (in the included header file).
In addition to all this, the validator is also claiming that it is not receiving code at all, which disables any chance of determining problems through that route.
The header and footer were both edited for these additions but are relatively long to include in this. The code used to edit were, for alexa <!-- tracking code here --> inserted into the head section of the header file, and the code for the footer was:
<span style="display:inline-block;width:160px;height:30px;text-align:center;border:#000 1px dotted;font-family:Arial,Helvetica,sans-serif;font-size:11px;background-color:#FFFFFF;"><strong style="display:block;padding:0px;margin:0px;">Reputation Management</strong>Submit Express</span>
If anymore code is needed to help then please let me know, I am not looking for someone to fix my problem but to give me relevant tips to help figure it out myself or if they do provide a fix to educate me as how they managed it so I may be able to use the same debugging method. Thanks!
EDIT: The issue, as discovered thanks to Jay, is "PHP Notice: Undefined index: HTTP_HOST in **/public_html/graphic-shack.com/index.php on line 40" and a google search has not revealed any effective results. This, to my untrained mind is jibberish, clarification would be greatly appreciated as line 40 is in fact a blank line, where as the text around it I can not post for some reason but will be viewable at http://graphic-shack.com/example.html

Answer 1

Open Cart uses UTF-8 throughout, so its definitely best to use that which your theme should set for the browser in the
The first thing I would ask is have you got error reporting set up (NOT OPEN CARTS ONE) that logs php errors?
If not, then you need to do something like
php_flag log_errors on
php_value error_log /path/to/custom/error.log
in your .htaccess file, so that you can log all your php errors to a file. this will need to be somewhere you can get the file, so change the path accordingly so you can view it.
Once you have that, you'll have a good base for working out the problem. I find it hard to believe that opencart has stopped because you added code. The most likely cause is a file missing, blanked or half written accidentally when saving or you've got some extra php code somewhere you shouldn't
== TEST.PHP CONTENT ==
<?php
echo '<pre>' . print_r($_SERVER, true) . '</pre>';
?>