I'm attempting to retrieve all the data from a table in MySQL and add it to an array.

I want to convert the blob into an encoded string and add it to an array with the other two columns and then return it as a JSON string. See below for my attempted code:
<?php
//Importing Database Script
require_once('dbConnect.php');
//Creating sql query
$sql = "SELECT * FROM picture";
//getting result
$r = mysqli_query($con,$sql);
//creating a blank array
$result = array();
//looping through all the records fetched
while($row = mysqli_fetch_array($r)){
//Pushing name and id in the blank array created
array_push($result,array(
"pictureID"=>$row['pictureID'],
"listingID"=>$row['listingID'],
"listingImage"=>$row [base64_encode('listingImage')]
));
}
//Displaying the array in json format
echo json_encode(array('result'=>$result));
mysqli_close($con);
The code is currently returning with a null value for the blob image.
"listingImage"=>base64_encode($row['listingImage'])