MySQL query result cannot be json encoded in PHP [duplicate]

I have the following PHP API which runs a MySQL query, when;

$sql = "select article_id, title, summary, image_url from articles limit 20"; 

I get the following result;

http://ec2-54-152-162-157.compute-1.amazonaws.com/list_view.php?user_id=1

the above is what I want the format of my output to look.

However when I change the $sql to below I get a blank screen;

$user_id = $_GET["user_id"];$sql = "select article_id, title, summary, image_url from articles where article_id in     (    select max(article_id) as last_article_id     from articles as a     join media_sources as ms     on a.rss_source_id = ms.media_source_id     where article_id not in         (        select article_id         from responses         where (activity_id = 1 and response = -1 and user_id = '1')         or (activity_id = 2 and user_id = '1')    )     group by category) limit 20;"$db = new  mysqli("remotehost", "user", "password", "db_name");$results = $db->query($sql);$articles["items"] = array();while($article = $results->fetch_assoc()){    $item = array ();    $item["article_id"] = $article['article_id'];    $item["article_title"] = $article['title'];    $item["article_summary"] = $article['summary'];    $item["article_image"] = $article['image_url'];    array_push($articles["items"], $item);    //echo json_encode($item);}echo json_encode($articles);

I've uncommented echo json_encode($item) in the 3rd from the bottom line of code; and put it in the below API. So there is data but I just can't get into the format I need.

http://ec2-54-152-162-157.compute-1.amazonaws.com/list_view2.php?user_id=1

**********************************EDIT******************************************

json_last_error_msg() returns:

Malformed UTF-8 characters, possibly incorrectly encoded. 

So I guess there are characters that can not be encoded in Json, I will have to investigate and strip them out. but still not sure why this one works as it is the same data getting encoded;

echo json_encode($item);