1. Same Origin Policy
2. Frame Load incomplete.

No. 1 is easy, if the two Top Level Domain aren’t the same, you cannot read the iframe data. Even for the same domain and different subdomains, one needs to write the following on the top of both client and the server:

 document.domain = "mytopleveldomain.com" 

No. 2

How to determine that the iframe has loaded completely in order to scan data off it ?
Well,

 <code>$(document).ready(function()</code>

will surely tell you if the page has loaded, but it won’t wait for the iframe(s) inside the parent document to finish loading, before returning true.

One may think, this might work:

$(document).ready(function() {

     $('#myIframe').ready(function() {
                // ALL CODE GOES HERE
     });
});

BUT, it doesn’t. The above code will tell you ONLY that the iframe loading has started, but won’t
tell you if its done.

Solution:
Well, the OLD window.onload to the rescue:

window.onload = function () {
    $('#myIframe').ready(function() {
                // ALL CODE GOES HERE
     });
};

http://www.mysite.com/users/images/user123/profile.jpg

Now, on changing this image, sometimes browsers don’t clear cache and thus show the last image. In Order to solve this, use a random number at the end of the image location and you will be able to see that image. as browsers will not cache it.

http://www.mysite.com/users/images/user123/profile.jpg?123

One method that I used is:

$stamp = time();
$imageLocation  = 'http://www.mysite.com/users/images/user123/profile.jpg?'.$stamp
echo $imageLocation

This should work.

Take a look what Rasmus is talking about AJAX

He follows the rule of simplicity as he described in his framework approach.

The very basic of ajax structure -

Javascript

function createRequestObject() {

var ro;

var browser = navigator.appName;

if(browser == “Microsoft Internet Explorer”){

ro = new ActiveXObject(“Microsoft.XMLHTTP”);

}else{

ro = new XMLHttpRequest();

}

return ro;

}

var http = createRequestObject();

function sndReq(action) {

http.open(‘get’, ‘rpc.php?action=’+action);

http.onreadystatechange = handleResponse;

http.send(null);

}

function handleResponse() {

if(http.readyState == 4){

var response = http.responseText;

var update = new Array();

if(response.indexOf(‘|’ != -1)) {

update = response.split(‘|’);

document.getElementById(update[0]).innerHTML = update[1];

}

}

}

HTML

<a href=”javascript:sndReq(‘foo’)”>[foo]</a>

<div id=”foo” />

server side script(rpc.php) -

switch($_REQUEST['action']) {

case ‘foo’:

/* do something */

echo “foo|foo done”;

break;

…

}

Thats all. Everything else is just building on top of this.

Source : http://news.php.net/php.general/219164