If you want to access the source of any files, you will need a system like Github but a private one since the XenForo code is under license. But for your addons, you can.
If you want to access some debug functions, there are some apps. Some display the html output and I've found one that has a js debugger but there were lags and didn't work that well. I can't tell you the name right now, but it's not that hard to find.
I do not want to have access to any php files. I want to look at something that displays the source code. One way is to make a screenshot from the source code and display it in iPhoto.
I do not want to do that, I want some more simple solution to just look at the code.
I also do not care about how the code will be displayed. Whether it is a PDF file I open in iBooks, or a HTML file I open in Safari. Or even an app like http://www.becomekodiak.com/kodiak-php.html It's also good enough to open a web front end like "Dreamweaver in the clouds" that displays the php content. I do not care whether the .php file is really on the device.
The iPad has no functionality like that. A 3rd party browser may implement it, but I highly doubt it. I would suggest learning and inspecting code on the computer where you have proper development tools.
It seems I did not make myself clear, I guess it is because I never really learned english.
I want a way to look at xenforos source code at my ipad. One way would be a program that converts all .php files into .html files. As an example, <html><body> would be translated into "<html><body><div style="color:green"><html></div><div style="color:blue"><body></div>". Then I upload all .html files onto my server domain.com/sourcecode/ and I have access to the .html files on my iPad once I open Safari (the web browser) and enter domain.com/sourcecode/.
These are editors - you wont find something that shows it as a 'book'. If you want that you'll need to convert the source to something like a PDF file. In which case something like this will do it: http://www.markkurossi.com/genscript/