json_encode on a PHP array returns a string

I've searched through the site and can't see a question quite like mine so I hope this isn't a copy.

So I've got a PHP script which is supposed to return a JSON array to my AJAX, and I want to use the array to generate a URL.

However, even though I'm pretty sure I've got an array on the PHP end, when I json_encode I'm getting a simple string out on the other end.

PHP Code:

    $n = 10;
    $all_titles = array();

    while($row = mysqli_fetch_array($result)) {
        $title = trim($row['Job Title']);
        if(array_key_exists($title, $all_titles)) {
            ++$all_titles[$title];
        } else {
            $all_titles[$title] = 1;
        }
    }

    arsort($all_titles);
    $top_titles = array_slice($all_titles, 0, $n);
    $title_list = array();

    foreach (array_values($top_titles) as $key => $val) {
        array_push($title_list, array_keys($top_titles)[$key]);
    }
    echo json_encode($title_list);

These array operations seem to be working so far, based on other tests I've done, so I'm pretty sure that $title_list is an array.

Here is my JS:

    xmlhttp.onreadystatechange=function() {
      if (xmlhttp.readyState==4 && xmlhttp.status==200) {
        alert("Generated URL: " + URL_gen(xmlhttp.responseText));
      }
    }

And finally where the problem arises:

function URL_gen(list) {
    var url = list[2];
    return url;
}

I have varied the number in list[#] (list[0], list[1], etc.) and each one is a character, meaning list (which is passed in from onreadystatechange as the responsetext from the PHP function above) is a string, not a JSON array.

Any suggestions?