I was always writing, that implementing a raytracer (or the better german description) in PHP would be far to slow. And then, two days ago, I had the feeling I should prove this ... this was also the way Image_3D was born. With the infrastructure Image_3D offers, all the existing models, the abstraction etc. it wasn't such a big thing. You mainly need to write a short algorithm which gets the intersection point for a polygon with a line, and that's it. For sure, you should optimze the algorithm a bit. And it is really fun to implement such a simple mathematical model like raytracing is.
He talks about some of the problems that raytracers don't have that normal renderers do (shadows, reflections, etc) and notes that the images display in the post took about seven minutes to render on his system. He proved that it can work, but notes that it supports his original hypothesis - rendering like this with PHP is just too slow.