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<?php

/** Setup WordPress */
require_once($_SERVER['DOCUMENT_ROOT'] . '/wp-load.php');

/** Set file path */
$file = $_SERVER['DOCUMENT_ROOT'] . '/wp-content/themes/liquid/import.json';

/** Remove existing files */
if (file_exists($file)) {
  unlink($file);
}

/** Move the uploaded file to the $file path */
move_uploaded_file($_FILES['import']['tmp_name'], $file);

/**
 * Take imported data and insert it into the database
 * Delete the import file after it's been imported
 */
if (file_exists($file)) {
  $contents = file_get_contents($file);

  $json = json_decode($contents);

  foreach($json->object_name as $item) {

    $user_id = get_current_user_id();

    /**
     * Setup Post Data here to be inserted into the database
     * include post type and meta values (Advanced Custom Fields)
     */
    $args= [
      'post_author' => $user_id,
      'post_content' => '',
      'post_title' => $item->json_key,
      'post_status' => 'publish',
      'post_type' => 'your_post_type',
      'meta_input' => [
        'custom_field_name' => $item->json_key 
      ]
    ];

    wp_insert_post($args);
  }

  unlink($file);

  echo "JSON Data was imported! :)";
} else {
  echo "Something went wrong...";
}

<div class="row">
  <div class="column">
    <form class="" action="/wp-content/themes/liquid/import.php" method="post" enctype="multipart/form-data">
      <input type="file" name="import" value="Upload JSON">
      <button type="submit" class="button">Upload</button>
    </form>
  </div>
</div>