Strange behaviour - No 'Access-Control-Allow-Origin' header is present

Question

I get the following error:

XMLHttpRequest cannot load www.mysite/ajax.php No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'www.mysite' is therefore not allowed access. The response had HTTP status code 404.

This occurs when I try to process an Ajax request.
This is fixed by adding:

header('Access-Control-Allow-Origin: http://www.example.com/');

However, this only works when I echo something (it can be anything) after this line:

<?php
  header('Access-Control-Allow-Origin: http://www.example.com/'); 
  echo "<span></span>"; //if this is removed, I'm presented with the error
  require_once 'functions.php';
  ...
?>

When the echo is emitted, I'm once again presented with the error.

Can anyone explain this peculiar behaviour?

EDIT

Could it have anything to do with the structure of my Ajax call?

var formdata = $( "#calculator_form" ).serialize();
    $.get( "ajax.php", formdata). done( function( data ) {
        $('.result').html(data); // Printing result into result class div
    });

EDIT 2

This can be found in the error_log:

malformed header from script. Bad header=<>... User unknown: php-cgi, referer: http://www.example.com/

NOTE - both ajax.php and index.php are on the same domain even though the responses and errors would suggest elsewise.

[Ways to circumvent the same-origin policy](http://stackoverflow.com/questions/3076414/ways-to-circumvent-the-same-origin-policy) — iam-decoder, Aug 13 '15 at 16:04
[How does Access-Control-Allow-Origin header work?](http://stackoverflow.com/questions/10636611/how-does-access-control-allow-origin-header-work) — iam-decoder, Aug 13 '15 at 16:09
Did you read my question fully? how does an `echo` following the `header()` make it work, but without the `echo` is breaks. — C_B, Aug 13 '15 at 16:11
[Access-Control-Allow-Origin: * not working?](http://stackoverflow.com/questions/9459949/access-control-allow-origin-not-working) — iam-decoder, Aug 13 '15 at 16:15
there's your problem: [jQuery AJAX cross domain](http://stackoverflow.com/questions/3506208/jquery-ajax-cross-domain) — iam-decoder, Aug 13 '15 at 16:19
How come if I echo something after the inclusion of `header()` it still works? — C_B, Aug 13 '15 at 16:25
possibly because if nothing is output on the screen it's not the default json request that ajax is asking for so the server throws a fit saying that it doesn't have json to give. — iam-decoder, Aug 13 '15 at 16:27
there's a lot of stuff after the `echo` , it needs to be directly after or it breaks. Baffling. — C_B, Aug 13 '15 at 16:28

Strange behaviour - No 'Access-Control-Allow-Origin' header is present

EDIT

EDIT 2

0 Answers0