BLOB字段返回null JSON

I am saving my images directly in the database (Yes I have read about that and know it's better to save the path to the image). The problem is selecting the image field which is a blob returns NULLto my JSON. I have alot read quite a few stackoverflow links of the same query but still not a clear explanation/ answer received has been able to help me. I have encoded using base_64 but when doing so nothing is showed on the webpage.

Would appreciate if I could be pointed in the correct direction.

This is part of my code:

header("Content-Type:application/json");
//select query



$rows = array();
while($r = mysqli_fetch_assoc($sth)) {
    $rows[] = $r;
}


    if (!empty($rows))
        {       
            $encode = array("StudentsList" => $rows);        
            //$json_response=json_encode($encode);
            $json_response=base64_encode($encode);
                    echo $json_response;
                    echo json_last_error(); //Returns 0
        }

if you need the json data please use json_encode

example

echo json_encode($json_response);

and also read php manual

http://php.net/manual/en/function.json-encode.php