Here's a submission from Paul Meagher:

Your readers might be interested in an article I wrote for IBM called Simple Linear Regression with PHP. I discuss some general guidelines for developing a math class and illustrate these ideas in the context of a Simple Linear Regression class I developed.

This article raises a point not seen in PHP much these days. In a time when most people are worried about database connections, and getting XSL or DOM to compile in, most haven't said anything about a "math library" being one of the missing elements. And, we're not talking simple stuff here - the "linear regression" talked about above is some pretty high-level stuff, but I can see why the author would miss it. So, if math's your bag, you really should check out this article, as well as the load of resources that he links to at the end....

Oh, and a small note: in the first sentence after listing 3 - it should read: "The SimpleLinearRegression class uses a least-squares criterion for deriving estimates of what the Y Intercept and Slope parameters should be. These estimated parameters are used to construct a linear equation (see Listing 3) to model the relationship between the X and Y values."