JavaScript code to make a JSONP call will as follows:

function common_jsonpRequest(url, name, query) {

 if (url.indexOf("?") > -1)
 url += "&jsonp="
 else
 url += name + "&";
 if (query)
 url += encodeURIComponent(query) + "&";
 url += new Date().getTime().toString(); // prevent caching

 var JSONPscript = document.createElement("script");
 JSONPscript.id = 'wordpressapi_jsonpRequest';
 JSONPscript.setAttribute("src", url + "&CacheBuster=" + Math.random());
 JSONPscript.setAttribute("type","text/javascript");

 // write the jsonp script tag
 document.body.appendChild(JSONPscript);
 //script.text = "";

}

function returncall (data){
 alert(data);
// you can use data as a variable also.
}

In same javascript you should write the returning function as above.

You can call JSONP this function as follows;

<a href="http://wordpressapi.com/files/JSONP-request.gif"><img class="alignnone size-medium wp-image-1167" title="JSONP-request" src="http://wordpressapi.com/files/JSONP-request-300x125.gif" alt="" width="300" height="125" /></a>
common_jsonpRequest(base_url+'getData.php?jsonp=mycallback&name=for_wordpressapi');

Notel: getData.php file will beahave like externaal javascript file so handle this file carefully.
getData.php sample file.

<?php
$data = "this is our dynamic data";
if($_REQUEST['name']=='for_wordpressapi'){

echo "returncall(".$data.")";

}

?> 

This will return the "this is our dynamic data" as a alert.