[urga]

a,b, op per javas***** global definieren und in $('button').click(function(e) {
befüllen. dann

....
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
});