json.js

// Run immediately
jQuery.getJSON('http://path.com/to/json.php?callback=?', function(html){
  // Make sure DOM is ready:
  jQuery(function($){
    $("#footer").before(html); // Example
  });
});

json.php

<?php ob_start(); ?>
<h1>Extra HTML content here</h1>
<?php
  $content = ob_get_contents();
  ob_end_clean();
  
  $content = json_encode( utf8_encode( $content ) );
  echo isset($_GET['callback']) ? $_GET['callback'] . '(' . $content . ');' : $content;