I understand what you are saying about using <input type="submit" name="opp" value="+"/> however any input type="submit" will run the form method and action once clicked.
Obviously when using a calculator the user will enter an x value then click one of the operation buttons (+, -, x, / etc) then they will enter their y value and click '=' (Currently my '=' button is input type="submit") if I make any other button submit it will send the data to early if you follow me?
I cannot use hidden input fields as the operation must be selected by the user.
I will upload the working version (which uses a drop down menu to select the operation) to the following link - Working calculator
Here is the version which isnt working with the operation selected by buttons - Button calculator
Thanks in advance.