Before you upload the files to the server, stop and think for a moment. Have you carried out any changes to the core PHP files of vBulletin? (We saw how to customize a bulletin board in Chapter 4.) If you have made any changes, uploading the new files will obliterate these changes. Most people want to keep any changes they made to their vBulletin installation, and this often keeps them from carrying out upgrades.
If there is only one change, then it may be easier just to make the same change again after the upgrade. However, if there are many changes, repeating them will be much more complicated and will need a lot more time and effort, and of course, there will always be scope to get things wrong. What you want is a file compare utility that highlights the differences between two files.
In Chapter 4, we saw how to modify the PHP code so that PayPal treats a subscription as a goods transaction. We will use this example to show how a file compare utility can be used to easily...