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PHP 101 (part 13): The Trashman Cometh
Secure your PHP scripts with clever input validation tricks.

| Waiting To Exhale |

Maybe you've heard the term GIGO before.

If you haven't, it stands for Garbage In, Garbage Out, and it's a basic fact of computer programming: if you feed your program bad input, you're almost certainly going to get bad output. And no matter which way you cut it, bad output is not a Good Thing for a programmer who wants to get noticed.

In case you think I'm exaggerating, let me give you a simple example. Consider an online loan calculator that allows a user to input a desired loan amount, finance term and interest rate. Let's assume that the application doesn't include any error checks, and that the user decides to enter that magic number, 0, into the Term field.

You can imagine the result. After a few internal calculations the application will end up attempting to divide the total amount payable 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 when designing the application.

The moral of this story? If you're serious about using PHP for web development, one of the most important things you must learn is how to validate user input and deal with potentially unsafe data. Such input verification is one of the most important safeguards a developer can build into an application, and a failure to do this can snowball into serious problems, or even cause your application to break when it encounters invalid or corrupt data.

That's where this edition of PHP 101 comes in. Over the next few paragraphs, I'm going to show you some basic tricks to validate user input, catch "bad" data before it corrupts your calculations and databases, and provide user notification in a gentle, understandable and non-threatening way. To prepare for this exercise, I suggest you spin up a CD of John Lennon singing "Imagine", fill your heart with peace and goodwill towards all men, and take a few deep, calming breaths. Once you've exhaled, we can get going.

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