Equation solver and plotter in PHP

I'm sure that everyone knows what an equation is. It's for example x^2=9. If I should solve an equation, I would write it down on a piece of paper and tried to edit that equation, everyone knows what is possible to do and what is not while editing. Or I could enter the equation into some computer program and it would solve it instead of me in a few seconds (miliseconds :p). There are loads of these programs on the internet, moreover some are online.

But, what I wanted to say or what does the heading refers to is that I tried to make an application which is able to solve an equation and print the graph with the solution. It's written in my favourite language PHP (which also means that the application runs online). The whole app is made by my own head so don't except something genuis, I'm sure it contains some bugs but it's still usable for me.

My app should be able to solve linear, quadratic and some goniometric (sine, cosine) equations and you can try it now! So enjoy and feel free to comment, I'm looking forward to it.