POST response is invalid. What could be?

Question

PHP

Hosting fell, after it was raised a bug appeared. When requesting a view:

$.post("/ajax/test", {country: 4}, function(res){
  console.log(res);
}, "JSON");

On the server I get a response like this:

var_dump($_POST);
----------------------
array(1) {
  ["country"] =>
         string(10) "4country=4"
}

I suspect that the problem is with the php setting, locally everything works as it should. What could be the problem? Thank you.
PS: found that changed php version from 5.3.27 to 5.4.38 (support changed)