Creating an AJAX/JSON callback function on the server

A JSONP response is basically just a piece of JavaScript which is a function call, with an argument being the JavaScript object or array for the data. You should look for a callback querystring variable to use as the function name.

A PHP example:

// the data to return
$data = array('image' => 'http://placehold.it/100x100');

// default callback name if none set
$function = 'callback';

// if callback set, override the default
if(isset($_GET['callback'])){
    // filter the callback so it can only contain a-zA-Z0-9
    $function = preg_replace("/[^a-zA-Z0-9]+/", "", $_GET['callback']);
}

// output
echo $function . '('  . json_encode($data) . ');';

Example when called via http://yoursite/data.php?callback=abc

abc({ "image" : "http://placehold.it/100x100" });

Consuming this service via jQuery would be:

$.ajax({
   url : 'http://yoursite/data.php?callback=?',
   dataType : 'jsonp',
   success :  function(response){
       console.log(response.image);
   }
});