XMLHttpRequest status 4 but responseText is empty

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I am trying to write a google ads like javaScript plugin. I can not use jQuery ajax as it should be working fine for all sites.Here is my JavaScript code.\[code\]var ajaxRequest; // The variable that makes Ajax possible!try{ // Opera 8.0+, Firefox, Safari ajaxRequest = new XMLHttpRequest();} catch (e){ // Internet Explorer Browsers try{ ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) { try{ ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP"); } catch (e){ // Something went wrong alert("Your browser broke!"); return false; } }}// Create a function that will receive data sent from the serverajaxRequest.onreadystatechange = function(){ if(ajaxRequest.readyState == 4){ alert( ajaxRequest.responseText); console.log('xhr',ajaxRequest) } else { //alert(ajaxRequest.readyState); //alert(ajaxRequest.responseText); }} var project_path = "http://www.domainname.com/"; //for stackoverflow, using right path in live code.var req_url = project_path + "ads/verifypublisher/";ajaxRequest.open("GET", req_url, true);ajaxRequest.send(null); \[/code\]And here is how my PHP file looks like, \[code\]echo("<b>hi:</b> ");exit();\[/code\]Yes, thats all i have there. Still responseText is empty. What am i missing here?