Output JSON string into PHPTemplate file

I want to output a JSON string as part of a data attribute on an HTML tag for a template in Drupal 7. For the life of me, I cannot figure out why it does not work (works perfectly in 8+).

I need to list a set of image urls and media sizes which is parsed by Javascript later.

In template.php:

    $variables['background_images'] = [];

    $variables['background_images'][] = [
      'srcset' => 'https://placeimg.com/1600/600/tech, https://placeimg.com/3200/1200',
      'media' => '(min-width: 100rem)',
    ];

    $variables['background_images'][] = [
      'srcset' => 'https://placeimg.com/1600/600/tech, https://placeimg.com/3200/1200',
      'media' => '(min-width: 48rem)',
    ];

    $variables['background_images'] = json_encode($variables['background_images']);

In the tpl file:

<div data-background="<?php print $background_images; ?>"></div>

The output is a mixed bag of issues in the browser. The URLs are escaped, and there are far more double quotes than I entered:

data-background="[{" srcset":"https:\="" \="" placeimg.com\="" 1600\="" 600\="" tech,="" https:\="" 3200\="" 1200","media":"(min-width:="" 100rem)"},{"srcset":"https:\="" 48rem)"}]"

I can't figure out why there is an extra spacing on the keys or why it escapes early and breaks the rest of it. It looks right (to me) with xdebug, but when its printed to the browser its wrong.

A second way I also tried (generate example data):

    $items = [];

    for ($x = 0; $x < 3; $x++) {
      $items[$x] = [
        'srcset' => 'https://placeimg.com/1600/600/tech, https://placeimg.com/3200/1200',
        'media' => '(min-width: 100rem)',
      ];
    }

    $variables['background_images'] = drupal_json_encode($items);