Just threw something together as practice. I'm wondering if you would code this differently. I know it's merely a few lines of code, but I'm sure there are people who can dissect it and code it more eloquently and efficiently than I have. I'm here to learn. Please pick it apart.
<p>This program will indicate the maximum number a numeric MySQL field can hold as determined by its length in bytes.</p>
Bytes: <input type="text" size="5" name="bytes" /><br />
<input type="submit" value="Calculate" />
There is a lot of code that you were using unnecessarily. No reason to use a constant, a function and definitely shouldn't be exiting an application for no reason (additionally, if you're using "exit" and don't have a message to send with it, no need to use exit(). :)
Exit stops the current php script. Since he's using procedural and not an object-oriented approach, it will not display anything after the exit. Otherwise you would be correct. Never the less an exit there would be kind of useless since nothing PHP follows it anyway.