When I was a student at universoty, I used to do lots of scientific PHP programs just for fun. Here is the one I really like. After 7 years I still can spend time playing with it and graw really cool graphs.

This PHP application takes 3 parametric expressions for x, y and z dimentions and draws the 3D graph representing the expressions. You can also save the result as an ordinary image. This program is really useful for students who like mathematics and algebra. 

t, is the variable which changes within the range you provide so foreach specific t, we will get a x, y and z values which will form our cool 3D graph

Here is a sample output:

You can try it here. Just make sure you type the expressions in C++ (PHP) format. This code can be downloaded here.