Jquery POST unicode string to codeigniter php

I'm trying to implement a custom suggestion engine using jquery.

I take the user input, call my codeigniter v2 php code in order to get matches from a pre-built synonyms table.

My javascript looks like this:

var user_str = $("#some-id").val();

$.ajax({
    type: "POST",
    url: "http://localhost/my-app/app/suggestions/",
    data: {"doc": user_str},
    processData: false
})
.done(function (data)
{
    // Show suggestions...
});

My PHP code (controller) looks like this:

function suggestions()
{
    $user_input = trim($_POST['doc']);
    die("[$user_input]");
}

But the data is NOT posted to my PHP :( All I get as echo is an empty [] (with no 500 error or anything)

I've spent 2 days looking for an answer, what I've read in SO / on google didn't help. I was able to make this work using GET, but then again, this wouldn't work with unicode strings, so I figured I should use POST instead (only this won't work either :()

Anyone can tell me how to fix this? Also, this has to work with unicode strings, it's an important requirement in this project.

I'm using PHP 5.3.8