Wordpress Json图像Feed

I'm trying to get a list of attached images, I know this script works cause I've used it already a couple times for tags and categories in the feed. When I use it with attachments, it will NOT include the separator line. I have no idea why. Is there so some limit on how many times you can use this? I've tried renaming things to and nothing happens.

"image": [ 
<?php
$output = '';
$separator = ",";
$attachments = get_children(array('post_parent' => $post->ID,
    'post_status' => 'inherit',
    'post_type' => 'attachment',
    'post_mime_type' => 'image',
    'order' => 'ASC',
    'orderby' => 'menu_order ID'));
if ($attachments) {
    foreach($attachments as $att_id => $attachment) {
    $output .= '"';
    $output .= wp_get_attachment_url($attachment->ID);
    $output .= '"';
                  }
    echo trim($output,$separator); 
}
?>
]

This is what I get.

"image": ["http://www.itsgametimela.com/wp-content/uploads/2015/10/sealed-deck-curve.png""http://www.itsgametimela.com/wp-content/uploads/2015/10/c4rd4rt_CsowW8oCu1.jpg"],

It will not toss in the comma to separate them. Any ideas?

After some digging came across this..

"image": [
<?php
$attachments = get_children(array('post_parent' => $post->ID,
         'post_status' => 'inherit',
         'post_type' => 'attachment',
         'post_mime_type' => 'image',
         'order' => 'ASC',
         'orderby' => 'menu_order ID'));
if ($attachments) {
        $output = array();
        foreach($attachments as $att_id => $attachment) {
        $output[] = '"'. wp_get_attachment_url($attachment->ID) . '"';
                  }
   $image_list = implode(",",$output);
   echo $image_list;
   }
   ?>
]

I'll leave this answer here for anyone else who wants to attempt this.