Help me properly make an AJAX call

I have a JavaScript function that is being called. I need to have it call a PHP function and return a true/false.

The script with the function is in the file /db/cancel_hike.php

My current JS looks like this:

function uncancelHike( hike_id )
{
    //var url = "/db/cancel_hike.php;
    var success = null;

    var request = window.ActiveXObject ?
          new ActiveXObject('Microsoft.XMLHTTP') :
          new XMLHttpRequest;

    request.open("GET", url , true);

    request.onreadystatechange = function()
    {
          if (request.readyState == 4)
          {
                var xmlDoc = request.responseXML;

                // obtain the array of markers and loop through it
                markers = xmlDoc.documentElement.getElementsByTagName("marker");

                for (var i = 0; i < markers.length; i++)
                {
                      // obtain the attribues of each marker
                      success = markers[i].getAttribute("success");

                  开发者_运维知识库    if ( success == "true" )
                      {
                        document.getElementById("success").style.display = 'block';
                        document.getElementById("warning").style.display = 'none';
                        document.getElementById("error").style.display = 'error';

                      }

                      if ( success == "false" )
                      {
                        document.getElementById("success").style.display = 'none';
                        document.getElementById("warning").style.display = 'none';
                        document.getElementById("error").style.display = 'block';
                      }
                }
          }
    }

    request.send(null);

    return false;
}

What I am having trouble with is:

1. How to call an actual function in the PHP script?
2. Do I absolutely need to have some XML returned? Or is there a way to just get back the returned value?
3. I am using YUI JS library. Do I need to make some calls to it, or is it not necessary in this case?

How to call an actual function in the PHP script?

You can't. You request URIs.

Write a PHP script that calls the function you want and place it at the URI you call.

(You can use query strings and the like as the input to an if statement that you use to conditionally call different functions)

Do I absolutely need to have some XML returned? Or is there a way to just get back the returned value?

You can return any kind of data you like.

I am using YUI JS library. Do I need to make some calls to it, or is it not necessary in this case?

It's a library. You never need to make calls to it. It often simplifies the code you have to write.

1. How to call an actual function in the PHP script?
2. Do I absolutely need to have some XML returned? Or is there a way to just get back the returned value?

Well, you don't call the actual function. What you want to do is pass variables using GET, that is, by appending them to the URL like file_name.php?var1=this&var2=that to pass var1 of "this" and var2 equaling "that." You retrieve them in the PHP file with $_GET['this'] and $_GET['that']. Whatever PHP outputs to the page via echo, print_r, etc. is then sent back in a request object as part of its responseText property.

You just set url in request.open to a URL on your site. For example, in your .js file:

request.open("GET", "answer_me.php?hike_id=" + hike_id, true);

And in your .php file:

<?php
$hike_id = $_GET['hike_id'];
if ($hike_id < 5) {
  echo "true";  // echo true would return "1", BTW
} else {
  echo "false"; // echo false would return nothing
}

Note that that will just return a string value to request.responseText of false, thus you could do this:

var result = request.responseText;

if (result === "true") {
  ...
  document.getElementById("success").style.display = "block";
  ...
} else {
  ...
  document.getElementById("success").style.display = "none";
  ...
}

You do not need it to be XML, especially as it looks like you're not really using the loop (the same three DOM elements are being assigned values each time).

And honestly, for AJAX I'd recommend using a framework like jQuery (or YUI, although I don't find its AJAX stuff as intuitive). Your entire code would look like this:

var $success = $("#success");
var $error = $("#error");

function cancelHikeCallback(data) {
  var is_success = (data === "true");
  $success.toggle(is_success);
  $error.toggle(!is_success);
}

function cancelHike(hikeIdToSend) {
  $.get("/db/cancel_hike.php", {hike_id: hikeIdToSend}, cancelHikeCallback);
}

IMO things like jQuery's $.ajax ($.get is a specialized form of $.ajax) make this stuff much easier to read and debug.

jsFiddle Example