How can i create a PHP calculator?

[Vagos]

I want to create a php calculator using html forms and ONE text box. My problem lies in the php file... if lets say the "add" button is pressed then it should add the current number to the one that already exists (either this is 0 if the current number is the first one or its a result from a previous calculation).I know there are easier ways to make a calculator but that's how i am asked to do it... thanks in advance!