Weird JSON encoding using json_encode

I’m using WordPress together with the JSON API Plugin (http://wordpress.org/extend/plugins/json-api/) to generate responses to our other site.

I’ve hit a really weird problem (we’re using PHP 5.3.6), when I pass the following array http://pastebin.com/xdfYjrvK to json_encode() it gives me this (with json content-type): http://pastebin.com/T61XGPP5

So the crap in the beginning, in the above example it’s 2609 and 0 in the end, it changes depending on the size of the response, more content -> higher hex number. It also appears only when the amount of response is “high enough”, so it works on small responses.

First I thought it was the plugin, but it works locally (on two different machines Mac OS X) and we’ve updated all packages on the VPS (Debian, Apache, Nginx, PHP) to the latest versions.

It’s only displayed when sending the content-type, not when outputting the $result with plain text instead of application/json:

$charset = get_option('blog_charset');
if (!headers_sent()) {
  header('HTTP/1.1 200 OK', true);
  header("Content-Type: application/json; charset=$charset", true);
}

echo $result;

$charset is set to utf-8.

The Google chrome console says: “Resource interpreted as Document but transferred with MIME type application/json.”

So, does anyone have a clue whats happening here?