你如何使用PHP和jquery获取jsonp？

I have this PHP:

$response = 'success';
$response = json_encode($response);
echo $_GET['callback'] . '('.$response.')';

and this jQuery:

$.ajax({url: "/*url*/?invite="+getinvite+"&api="+api, dataType:"jsonp", 
    success: function(response){/*do stuff*/}});

On XAMPP on my laptop it was working great. Then when I put it on my server it gives me this error: Undefined index: callback.

Why is it not letting me set $_GET['callback']? and how would this problem be fixed?

You aren't setting a parameter called 'callback' and so it isn't set in the $_GET variables.

You could fix the error message by doing:

$callback = "";
if(array_key_exists('callback', $_GET) == TRUE){
    $callback = $_GET['callback'];
}

However I don't think you may have misunderstood the .ajax call in jQuery - the javascript function that is called on success is not passed to the server; it is only held inside jQuery.

To make your ajax call simple, use getJson method designed specifically for JSONP response.

http://api.jquery.com/jQuery.getJSON/

This will automatically create a callback function name which will be passed as parameter to your jsonp api . example from the page :

$.getJSON("http://api.flickr.com/services/feeds/photos_public.gne?jsoncallback=?",
  {
    tags: "cat",
    tagmode: "any",
    format: "json"
  },
  function(data) {
    $.each(data.items, function(i,item){
      $("<img/>").attr("src", item.media.m).appendTo("#images");
      if ( i == 3 ) return false;
    });
  });