Getting google plus shares for a given URL in PHP

I want to get the number of shares on google plus for a given URL in PHP. I found this function to do that:

function get_shares_google_plus($url) {
  $curl = curl_init();
  curl_setopt($curl, CURLOPT_URL, "https://clients6.google.com/rpc");
  curl_setopt($curl, CURLOPT_POST, 1);
  curl_setopt($curl, CURLOPT_POSTFIELDS, '[{"method":"pos.plusones.get","id":"p","params":{"nolog":true,"id":"' . $url . '","source":"widget","userId":"@viewer","groupId":"@self"},"jsonrpc":"2.0","key":"p","apiVersion":"v1"}]');
  curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
  curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-type: application/json'));
  $curl_results = curl_exec ($curl);
  curl_close ($curl);
  $json = json_decode($curl_results, true);
  print_r($json);
  return intval( $json[0]['result']['metadata']['globalCounts']['count'] );
}

However, I always get the same message: Notice: Undefined index: result in ....

I make print_r($json), and I get: Array ( [0] => Array ( [error] => Array ( [code] => 400 [message] => Invalid Value [data] => Array ( [0] => Array ( [domain] => global [reason] => invalid [message] => Invalid Value ) ) ) [id] => p ).

Any suggestions?

The RPC API was never meant for public use and Google changed authentication in order to prevent abuse. Thus, the code that you posted does not work any longer. However, I found a much simpler solution:

Update (23.01.2013): Google blocked this URL in december 2012 - so this method doesn't work any longer!
Update (15.05.2013): The method works again!

<?php
/**
 * Get the numeric, total count of +1s from Google+ users for a given URL.
 * @author          Stephan Schmitz <eyecatchup@gmail.com>
 * @copyright       Copyright (c) 2013 Stephan Schmitz
 * @license         http://eyecatchup.mit-license.org/  MIT License
 * @link            <a href="https://gist.github.com/eyecatchup/8495140">Source</a>.
 * @param   $url    string  The URL to check the +1 count for.
 * @return  intval          The total count of +1s.
 */
function getGplusShares($url) {
    $url = sprintf('https://plusone.google.com/u/0/_/+1/fastbutton?url=%s', urlencode($url));
    preg_match_all('/{c: (.*?),/', file_get_contents($url), $match, PREG_SET_ORDER);
    return (1 === sizeof($match) && 2 === sizeof($match[0])) ? intval($match[0][1]) : 0;
}

Update (18.01.2014): Here's an improved version that uses curl, a fallback host and does some error handling (the latest version can be found here https://gist.github.com/eyecatchup/8495140).

<?php
/**
 * GetPlusOnesByURL()
 *
 * Get the numeric, total count of +1s from Google+ users for a given URL.
 *
 * Example usage:
 * <code>
 *   $url = 'http://www.facebook.com/';
 *   printf("The URL '%s' received %s +1s from Google+ users.", $url, GetPlusOnesByURL($url));
 * </code>
 *
 * @author          Stephan Schmitz <eyecatchup@gmail.com>
 * @copyright       Copyright (c) 2014 Stephan Schmitz
 * @license         http://eyecatchup.mit-license.org/  MIT License
 * @link            <a href="https://gist.github.com/eyecatchup/8495140">Source</a>.
 * @link            <a href="http://stackoverflow.com/a/13385591/624466">Read more</a>.
 *
 * @param   $url    string  The URL to check the +1 count for.
 * @return  intval          The total count of +1s.
 */
function GetPlusOnesByURL($url) {
    !$url && die('No URL, no results. ;)');

    !filter_var($url, FILTER_VALIDATE_URL) &&
        die(sprintf('PHP said, "%s" is not a valid URL.', $url));

    foreach (array('apis', 'plusone') as $host) {
        $ch = curl_init(sprintf('https://%s.google.com/u/0/_/+1/fastbutton?url=%s',
                                      $host, urlencode($url)));
        curl_setopt_array($ch, array(
            CURLOPT_FOLLOWLOCATION => 1,
            CURLOPT_RETURNTRANSFER => 1,
            CURLOPT_SSL_VERIFYPEER => 0,
            CURLOPT_USERAGENT      => 'Mozilla/5.0 (Windows NT 6.1; WOW64) ' .
                                      'AppleWebKit/537.36 (KHTML, like Gecko) ' .
                                      'Chrome/32.0.1700.72 Safari/537.36' ));
        $response = curl_exec($ch);
        $curlinfo = curl_getinfo($ch);
        curl_close($ch);

        if (200 === $curlinfo['http_code'] && 0 < strlen($response)) { break 1; }
        $response = 0;
    }
    !$response && die("Requests to Google's server fail..?!");

    preg_match_all('/window\.__SSR\s\=\s\{c:\s(\d+?)\./', $response, $match, PREG_SET_ORDER);
    return (1 === sizeof($match) && 2 === sizeof($match[0])) ? intval($match[0][1]) : 0;
}

Update (02.11.2017): The +1 count is officially dead! As announced in this Google+ Post by Product Manager John Nack Google recently removed the share count (aka +1 Count) from their web Sharing buttons. (They claim that the purpose of this move is for making the +1 button and share box load more quickly.)