Logic errors occur when an application runs perfectly as far as the PHP engine

Logic Errors

Logic errors occur when an application runs perfectly as far as the PHP engine

is concerned, but the code does something other than what you had intended.

For example, imagine you have a mailing script that you want to use to send the

same message to a few of the members of your online forum. To your horror, you

discover upon executing the script that you've mailed the entire forum membership

… twenty times!

These kinds of problems are the most difficult to find; users of Windows XP will

be well acquainted with Windows updates—even big companies struggle with

logic errors.

Critical to finding logic errors is your ability to test rigorously your code in a safe

environment that's separate from your “live” Web server. Thankfully, PHP and

related technologies like Apache and MySQL (if you're using them) are cross

platform, which makes putting together an effective development environment

easy even if the underlying operating systems are different.

You should also investigate unit testing, a facet of Extreme Programming (XP),

to which you'll find an introduction in Volume II, Chapter 6. I've also suggested

further reading at the end of this chapter.

In Chapter 10, I'll be taking a look at strategies for handling errors themselves,

particularly environment errors. In particular, we'll discuss how you can record

(or trap) errors for your analysis without displaying ugly messages to your applications

users.