Sample using very basic XML feed file caching

json.php

<?php
/*
 * File to convert NextBus' XML Feeds to JSON format
 * 	that our script will use.
 */

// Get route from $_GET['r'] variable, or set to default
// TODO: Add in check for valid route
$route = (isset($_GET['r']) && (int) $_GET['r'] != 0) ? (int) $_GET['r'] : '501';

$filename = "cache/vehicleLocations.".$route.".xml";

// Cache data for X seconds
if (!file_exists($filename) || ((time() - filemtime($filename)) >= 60*60*24)) {
  $file_contents = file_get_contents("http://webservices.nextbus.com/service/publicXMLFeed?command=vehicleLocations&a=ttc&r=" . $route . "&t=0");
  file_put_contents($filename, $file_contents);
}

$vehicle_locations_xml = simplexml_load_file($filename);

//print_r($vehicle_locations_xml);

$vehicles = array();

foreach ($vehicle_locations_xml->vehicle as $vehicle) {
  //print_r($vehicle);
  /* Set a simpler direction tag with "N", "S", "E", or "W".
   * April 6, 2011 - Direction tags now look like this: 510_0_510A
   * Depending on the route (east/west or north/south route), dir tag means:
   * _1_ is Westbound OR Northbound
   * _0_ is Eastbound OR Southbound
   * 
   * 510 Spadina & 511 Bathurst are only North/South Routes
   * The rest are East/West
   */
  $dirTag = (string) $vehicle['dirTag'];
  $dirTagParts = explode("_", $dirTag);
  
  if (is_array($dirTagParts) && count($dirTagParts) > 1) {
    $direction_num = (int) $dirTagParts[1]; // 1 or 0
    $routeSub = $dirTagParts[2];
  } else {
    $direction_num = null;
    $routeSub = null;
  }

  switch($route) {
    case 510:
    case 511:
      if ($direction_num === 0)
        $direction = "S";
      else if ($direction_num === 1)
        $direction = "N";
      else
        $direction = "null"; //default
      break;
    case 501:
    case 502:
    case 503:
    case 504:
    case 505:
    case 506:
    case 507:
    case 508:
    case 509:
    case 512:
    default:
      if ($direction_num === 1)
        $direction = "W";
      else if ($direction_num === 0)
        $direction = "E";
      else
        $direction = "null"; //default
      break;
  }

  $vehicles[] = array(
    // Need to cast attributes to a (string), else it will be treated as an object
    'id'      			=> (string) $vehicle['id'],
    'lat'     			=> (string) $vehicle['lat'],
    'lng'     			=> (string) $vehicle['lon'],
    'dirTag'        => $dirTag,
    'routeSub' 			=> $routeSub,
    'dir'     			=> $direction,
    'heading' 			=> (string) $vehicle['heading'],
    'secsSinceReport' 	=> (string) $vehicle['secsSinceReport']
    );
}

$vehicles_json = json_encode(array("vehicles" => $vehicles));

echo $vehicles_json;

?>