simple jsonp not working with php server - php

Question

I tried following some basic examples, and it is not working. I am not sure I completely understand jsonp, but I followed the basic tutorials and on an intuitive level, I can't see anything wrong. (Of course, I am missing something, hence the question).
JavaScript code:
postData = $(this).serialize();
console.log(postData);
$.ajax({
data: postData,
url: 'externaldomain.php',
dataType: 'jsonp',
success: function(data){
console.log(data);
alert('Your comment was successfully added');
},
error: function(){
console.log(data);
alert('There was an error adding your comment');
}
});
PHP code:
$tag = mysql_real_escape_string($_GET["callback"]);
The annoying part is that it is not even showing me an error to Google for.
Can anyone help out figuring the problem please?

Answer 1

Since you haven't posted your full or relevant PHP code I'm going to assume it looks like this
$tag = mysql_real_escape_string($_GET["callback"]);
echo $tag."(";
// print json here
echo ")"
I'm not really sure how jquery handles jsonp requests but what I used to do is add a new script tag to the DOM that looks like this
<script> function cb(json) { alert(json); } </script> // (I)
<script src="{url}?callback=cb"></script> // (II)
When (II) is loaded, and because we have a callback then the script that we are including in the DOM looks like this
cb({
// json goes here
})
which pretty much is like a function call to some function called cb that we are using. It is therefore natural to have an implementation of cb (which is in (I)). That implementation is similar to the success(data) function in jQuery's ajax. It is used to use and manipulate the json requested
However I noticed that you are doing a POST request along the ajax call with jsonp. Unfortuantely (according to this question How to make a jsonp POST request that specifies contentType with jQuery?) doing POST with JSONP is not feasable due to implementation pursposes.

Answer 2

You can't POST using JSONP... it simply doesn't work that way...
So you can usually only perform GET requests. You can NOT perform POST requests.
For details about how jsonp work look at this nice article. jsonp-how-does-it-work
For more clearification :
See Other SO question Link1 Link 2
But there is work around for that with some limitations look at this Using PUT/POST/DELETE with JSONP and jQuery. (for me nothing works at all)