a,b, op per javas***** global definieren und in $('button').click(function(e) {
befüllen. dann
Code:
....
var param1 = 12;
var param2 = 13;
var operator = '+';
....
$("#addieren").click(function(){
$.ajax({
url: rechnen.php,
dataType: 'json',
data: {a: param1, b: param2, operator: operator},
success: callback
});