The new Census Geocoding API (http://geocoding.geo.census.gov/geocoder/) is really great, but doesn't accept cross-site AJAX request. This is a simple PHP wrapper to passes javascript requests to the Census.

api.php

<?php
require 'census-geocoder-lib.php';

// Who doesn't want to send in coordinates or addresses, and get Census geography back?

try {
  // try passing ?lat=36.54&lng=-86.5&returntype=geographies&searchtype=coordinates
  $api = new census_geocode($_GET);
  echo $api->run();

  /*
  // another way, specifying the returntype and searchtype:
  // try passing ?returntype=locations&searchtype=onelineaddress&address=1600+Pennsylvania+Avenue+Washington+DC  
  $api2 = new census_geocode($_GET);
  echo $api2->run();
  */
} catch (Exception $e) {
  print $e->getmessage();
}

census-geocoder-lib.php

<?php
/**
* Census Geocoder wrapper
*
* see http://geocoding.geo.census.gov/geocoder/Geocoding_Services_API.pdf for details on the census api
* One difference is that this accepts `lat` and `lng` as inputs for coordinates, whereas the census wants x and y.
* If you are like me you have a hard time thinking of the globe as graph paper, so `lat` and `lng` are better
*/
class census_geocode {
  // These are all of the string settings. Lat and Lon (x and y) are assigned especially, because they are floats.
  private $params = array(
    'vintage'   => 'Census2010_Census2010',
    'benchmark' => 'Public_AR_Census2010',
    'format'    => 'json',
    'layers'    => NULL,
    'address'   => NULL,
    'street'    => NULL,
    'city'      => NULL,
    'state'     => NULL,
    'zip'       => NULL
  );
  private $returntypes = array('geographies', 'locations');
  private $searchtypes = array('onelineaddress', 'address', 'coordinates');
  private $url = 'http://geocoding.geo.census.gov/geocoder/%1$s/%2$s';
  public $encoded;

  /**
    * @param $raw array A set of parameters, perhaps raw $_GET data
    * @param $curlopts array Optional arguments for php libcurl
    * @param $url string The census API endpoint
  */
  function __construct($raw, $curlopts=array(), $url=false) {
    $this->url = ($url) ? $url : $this->url;
    $this->curlopts = $curlopts;

    // the basic options in the geocoder aren't set as GET options, instead the url itself changes
    $this->url = $this->set_geocoder_type($raw);

    // encode the url parameters
    $this->encoded = $this->encode_params($raw);
  }

  function set_geocoder_type($raw) {
    if (array_key_exists('returntype', $raw) && in_array($raw['returntype'], $this->returntypes)):
      $this->returntype = $raw['returntype'];
    endif;

    if (array_key_exists('searchtype', $raw) && in_array($raw['searchtype'], $this->searchtypes)):
      $this->searchtype = $raw['searchtype'];
    endif;

    if ($this->returntype && $this->searchtype):
      return sprintf($this->url, $this->returntype, $this->searchtype);  
    else :
      throw new Exception('Need both a returntype and a searchtype', 1);
    endif;
  }

  function encode_params($raw) {

    if ($this->searchtype === 'coordinates') :
      if (!array_key_exists('lat', $raw) || !array_key_exists('lng', $raw)) :
        throw new Exception('Need both a lat and a lng', 1);
      endif;

      // assign numeric versions of the x and y
      $this->params['x'] = floatval($raw['lng']);
      $this->params['y'] = floatval($raw['lat']);
    endif;

    // start off the encoded string
    $encoded = '?';

    foreach ($this->params as $key => $value):
      $passed_val = (array_key_exists($key, $raw)) ? strval($raw[$key]) : NULL;

      $value = ($passed_val) ? $passed_val : $value;
      
      if ($value) :
        $encoded .= urlencode($key) .'='. urlencode($value) . '&';
      endif;
    endforeach;

    return substr($encoded, 0, -1);
  }

  function run() {
    if (!isset($this->encoded)):
      throw new Exception('Missing encoded arguments', 1);
    endif;

    $handle = curl_init($this->url . $this->encoded);
    curl_setopt($handle, CURLOPT_RETURNTRANSFER, TRUE);
    
    if ($this->curlopts):
      curl_setopt_array($handle, $this->curlopts);
    endif;
    
    $result = curl_exec($handle);

    if ($result === false):
      throw new Exception(curl_error($handle), 1);
    endif;

    curl_close($handle);
    return $result;
  }
}