Implementing Google Custom Search

In the case that you weren't already aware there is the following: http://drupal.org/project/google_cse

Great guide. For anyone looking for a Drupal 7 version, I updated the mygooglecse.module (and .info file to version 7) so it would work in D7 (hook_block and hook_perm both needed updating). I've left the original code commented out so the changes would make more sense.

<?php
/**
* My Google Custom Search Engine
* 
* This search is set up to work with the two page search option. See: www.google.co.uk/cse/
* 
*/
function mygooglecse_block_info() {
  // This example comes from node.module.
  $blocks['google_custom_search'] = array(
    'info' => t('My Google Custom Search'), 
    'cache' => DRUPAL_NO_CACHE,
  );
  return $blocks;
}


function mygooglecse_block_view($delta = '') {
  // This example is adapted from node.module.
  //$mymodule_testblock_content = '<gcse:searchbox-only></gcse:searchbox-only>';
  $block['title'] = t('');
  //drupal_add_js(drupal_get_path('module', 'mygooglecse') . '/mygooglecse.js', 'module', 'header');
  //$block['content'] = '<gcse:searchbox-only></gcse:searchbox-only>';
  $block['content'] = array(
    '#markup' => '<gcse:searchbox-only queryParameterName="as_q"></gcse:searchbox-only>',
    '#attached' => array(
        'js' => array(
          drupal_get_path('module', 'mygooglecse') . '/mygooglecse.js',
      ),
    ),
  );
  return $block;
     
}

/**
 * Implementation of hook_block().
 */
/*function mygooglecse_block($op = 'list', $delta = 0, $edit = array()) {
  if ($op == 'list') {
    $blocks['google_custom_search']['info'] = t('My Google Custom Search');
   // Note, we can't cache this block as it will be set up to include some javascript
   // in the head of the page if the block is called from the cache this javascript 
   // won't be included.
    $blocks['google_custom_search']['cache'] = BLOCK_NO_CACHE;
    return $blocks;
  }
  else if ($delta == 'google_custom_search' && $op == 'view' && user_access('use my google custom search')) {
    $block['title'] = t('Search');
    drupal_add_js(drupal_get_path('module', 'mygooglecse') . '/mygooglecse.js', 'module', 'header');
    // Note, the following comes from the first element code box on Google, but 
    // we have set the query parameter name here so it doesn't interfere with Drupal. 
    // See: libretechtips.com/tips-drupal7-drupal6/how-use-google-cse-drupal
    $block['content'] = '<gcse:searchbox-only queryParameterName="my_gcs"></gcse:searchbox-only>';
    return $block;
  }
}*/

// We set up a permission to use the search. I did this mainly so that I could do 
// some final testing on the production site with admin access only first to
// check that I had set it up correctly (i.e. you will need to remember to change
// the url of your search results page on your Google search engines settings
// page to the url for the production site.
/*function mygooglecse_perm() {
  return array('use my google custom search');
}*

/**
 * Implements hook_permission().
 */
function mygooglecse_permission() {
  return array(
    'access mygooglecse_searchpage content' => array(
      'title' => t('Use google custom search'),
    )
  );
}

/**
* Implementation of hook_menu()
* 
*/
function mygooglecse_menu() {
  // Here we set up the page where the results will display:
  $items['search-results'] = array(  // Change this url to wherever you want the page to be
    'title' => 'Search ',
    'description' => 'My Google Custom Search',
    'page callback' => 'mygooglecse_searchpage', // The function that will be called to set up the search page
    'access arguments' => array('Use google custom search'), // Those with this permission will be allowed to see the results
    'type' => MENU_NORMAL_ITEM,
  );

  return $items;
}

// Here we set up the page:
function mygooglecse_searchpage() {
  // puts the call to your javascript file containing the Google code in the header
  // of the page:
  //drupal_add_js(drupal_get_path('module', 'mygooglecse') . '/mygooglecse.js');

  // Now create the content of your page:
  $content = '';
  // This is the search box again - so that it also appears at the top of the search
  // page.
  $content .= '<gcse:searchbox-only queryParameterName="as_q"></gcse:searchbox-only>';
  // Drupal search link:
  // This creates a link to the Drupal standard search from the Google results page:
  //$content .= l(t('Advanced search'), 'search/node/' . $_GET['as_q']);
  // This is the code for the results list.
  // Note we also add the queryParameterName setting here to the code. See
  // developers.google.com/custom-search/docs/element#supported_attributes
  // for a full list of supported attributes.
  // I've also set it so links to pages in the results open in the same window rather
  // than a new tab/window which is the default.
  $content .= '<gcse:searchresults-only queryParameterName="as_q" linkTarget="_self"></gcse:searchresults-only>';
  return $content;
}
?>

Sweet write up and explanation!
The Drupal 7 version has a minor problem though.
The malformed call to hook_permissions leads to access denied error for anonymous users (although it appears correctly in the user permissions list ).

Replace with the following code in order to have proper permission control over the display of the page

/**
* Implementation of hook_menu()
*/
function mygooglecse_menu() {
  // Here we set up the page where the results will display:
  $items['search-results'] = array(  // Change this url to wherever you want the page to be
    'title' => 'Search ',
    'description' => 'My Google Custom Search',
    'page callback' => 'mygooglecse_searchpage', // The function that will be called to set up the search page
    'access arguments' => array('use_google_custom_search'), // Those with this permission will be allowed to see the results
    'type' => MENU_NORMAL_ITEM,
  );
  return $items;
}

function mygooglecse_permission() {
  return array(
    'use_google_custom_search' => array(
      'title' => t('Use google custom search'),
      'description' => t('Who can have access to the google search results page.'),
    ),
  );
}

Thank you both, you made my day :)

Nice article, just wanted to point out that there is also the more powerful (but not free) Google Site Search branch, that has a nice Drupal module you can use https://drupal.org/project/gss .
Main reason we ended up choosing this over GCS was more customization for the results, ie. letting users filter the results by date, content type, taxonomy etc. Was not that easy to figure it all out, if anyone ever needs it then a little help here http://ardihundt.com/2013/09/15/filtered-google-site-search-for-drupal/

Maybe I am dumb but there were a couple things not obvious to me that took me awhile to figure out.

On the google website http://www.google.com/cse/ you have to set search results details to match the settings in these files.

So the way these files are shown here, on the google end you have to set the query parameter to "as_q" (google default is "q") and the url to "http://yourwebsite.com/search-results".

Seems obvious now but it wasn't at first from reading the instruction provided here over and over again.