这段代码如何生成正确的字符序列？

The code in question comes from MathGuard, a PHP anti-spam CAPTCHA script that requires the user to answer a simple math problem. It displays the digits and operator symbols as 3x5 matrices of random characters. I understand how the code works in the sense that I can follow the code and understand what it's doing; I just don't understand how one would come to this solution., a PH