can not pass json array from PHP to JQuery -

i have been trying pass simple array php jquery. here have far relevant functions in html/js:

function myquery() {   var xmlhttp = new xmlhttprequest();  var url = "sandbox.php";   xmlhttp.onreadystatechange=function() {   if (xmlhttp.readystate == 4 && xmlhttp.status == 200) {    myfunc(xmlhttp.responsetext);   }  }  xmlhttp.open("get", url, true);  xmlhttp.send();  }  function myfunc(response) {  var arr = json.parse(response);  document.getelementbyid("demco").innerhtml = arr[0]; } 

for php have tried:

<?php  $arr = [4,7,2];  echo json_encode($arr); ?> 

and:

<?php  $arr = array(4,7,2);  echo json_encode($arr); ?> 

according debugger fails @ json.parse(response) every time. wrong in code here? tried running array, [4,7,2], through http://json.parser.online.fr/ , got no issues. i'm @ wits end, me se!

edit:

from console: parse error in php on line x x line containing: $arr = [4,7,2]; uncaught exception: syntaxerror: json.parse: unable parse value:

error thrown @ line y, column 2 in myfunc(response) in http://... var arr = json.parse(response); called line z, column 4 in () in http://localhost/sandbox.html: myfunc(xmlhttp.responsetext);

kindly set output page header application/json clearly. required make output in json code.

<?php header('content-type:application/json'); $arr = array(4,7,2); echo json_encode($arr); 

this make pure json output , readable javascript