你如何从javascript函数调用php函数并同时传递一个变量？

Right now I have this:

现在我有这个：

  var ajax = getRequest();
  ajax.onreadystatechange = function(){
      if(ajax.readyState == 4){
          document.getElementById('results').innerHTML = ajax.responseText;
      }
  }
  ajax.open("GET", "get_schedule.php", true);
  ajax.send(null);
  }

        function getRequest() {
        var req = false;
        try{
            // most browsers
            req = new XMLHttpRequest();
        } catch (e){
            // IE
            try{
                req = new ActiveXObject("Msxml2.XMLHTTP");
            } catch (e) {
                // try an older version
                try{
                    req = new ActiveXObject("Microsoft.XMLHTTP");
                } catch (e){
                    return false;
                }
            }
        }
        return req;
    }
  var ajax =