Building An Extensible Form Validator Class
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One of my most common activities during the development cycle for a Web application involves writing code to validate the data entered into online forms.
This might seem like a trivial task, but the reality - as anyone who's ever spent time developing a robust Web application will tell you - is completely different. Data verification is one of the most important safeguards a developer can build into an application that relies on user input, and a failure to build in this basic error checking can snowball into serious problems (and even cause your application to break) if the data entered is corrupt or invalid.
In order to illustrate this, consider a simple example, culled from my own experience: an online loan calculator that allows a user to enter the desired loan amount, finance term and interest rate. Now, let's assume that the application doesn't include any error checks. And let's also suppose that the user decides to enter that magic number, 0, into the term field.
I'm sure you can imagine the result. The application will perform a few internal calculations that will end in it attempting to divide the total amount payable by the specified term - in other words, division by zero. The slew of ugly error messages that follow don't really bear discussion, but it's worth noting that they could have been avoided had the developer had the foresight to include an input-validation routine while designing the application.
The things about data validation, though, is that it's one of those things you can't hide from. Even if you develop and release a Web application without building in any validation routines (either through ignorance or laziness), you can be sure that your customer's going to demand a fix for it in the next release of the software. And since - as I said right at the beginning of this article - it's one of the few things you're likely to do over and over again when building Web applications, it's worthwhile spending a little time to make the process as painless as possible.
That's where this article comes in. Over the next few pages, I'll be attempting to build a reusable library of functions for form input validation, in an attempt to save myself (and, hopefully, you) some time the next time an application needs to have its input checked for errors. The end result of this experiment will be a PHP class that can be easily included in your scripts, and that exposes basic object methods for data validation. It may not meet *all* your needs; however, the process should be instructive, especially if you're new to object programming in PHP, and you'll have very little difficulty customizing it so that it works for you.
Let's get going!
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