Simple GPlus Avatar Exchange the Gravatar avatar with the GooglePlus avatar if an user provide a link to his GooglePlus profile.

index.php

<?php
/**
 * WordPress-Plugin Simple GPlus Avatar
 *
 * PHP version 5.3
 *
 * @category   PHP
 * @package    WordPress
 * @subpackage Simple GPlus Avatar
 * @author     Ralf Albert <me@neun12.de>
 * @license    GPLv3 http://www.gnu.org/licenses/gpl-3.0.txt
 * @version    0.1.20130101
 * @link       http://wordpress.com
 */

/**
 * Plugin Name:	Simple GPlus Avatar
 * Plugin URI:	http://yoda.neun12.de
 * Description:	Replacing the standard avatar in comments with a GPlus avatar if a user's GPlus profile url is provided
 * Version: 	0.1.20130101
 * Author: 		Ralf Albert
 * Author URI: 	http://yoda.neun12.de
 * Text Domain:
 * Domain Path:
 * Network:
 * License:		GPLv3
 */

/*
 This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
*/

/*
 * You can configure some parts of the plugin e.g. in your functions.php
 *
 *  - GooglePlus API-Key
 *      add_filter( 'simple_gplus_avatar_apikey', function () { return 'YoUrDaMnEdGoOgLePlUs_API_KEY'; } );
 *        You have to get an api-key for the G+ API (https://code.google.com/apis/console/)
 *        And you have to setup this api-key with apply_filters()!
 *        GooglePlus does not allow anonymous access to their API
 *
 *  - Use an extra field
 *  	add_filter( 'simple_gplus_avatar_use_extrafield', '__return_false' );
 *		  Use '__return_false' if no extra field should be used.
 *		  Default is: true; use an extra field
 *
 * - Label text
 * 		apply_filters( 'simple_gplus_avatar_labeltext', function () { return __( 'Your translated label text', YourTextDomain ); } );
 * 		  If an extrafield is used, overwrite the defualt label text
 * 		  Default is: Facebook Profile URL
 *
 *  - Alternate text for images
 *  	apply_filters( 'simple_gplus_avatar_alttext', function () { return $default_alternate_text; } );
 *  	  Setup an default alternate text for images
 *  	  Default is: Facebook Avatar
 */



/**
 * Main namespace for Simple GPlus Avatar
 * @author Ralf Albert
 *
 */
namespace SimpleGPlusAvatar;
/*
 * THIS IS F****** IMPORTANT!!!ONEELEVEN
 * YOU HAVE TO SETUP YOUR API KEY WITH add_filter( 'simple_gplus_avatar_apikey', function () { return YOUR_API_KEY } );
 * Do it here or in your functions.php (or write a plugin to store your api-key in the database)[or try some Voodoo]
 */
//add_filter( 'simple_gplus_avatar_apikey', function () { return 'YoUrDaMnEdGoOgLePlUs_API_KEY'; } );


/**
 * Initialize plugin on theme setup.
 * This is a theme specific functionality, but the code store some data
 * in the comment meta. This data can be better removed on plugin uninstall.
 *
 */
add_action(
	'after_setup_theme',
	__NAMESPACE__ . '\plugin_init',
	10,
	0
);

register_uninstall_hook(
	__FILE__,
	__NAMESPACE__ . '\uninstall'
);

/**
 * Initialize the plugin, hook up all actions and filters
 * @uses apply_filters() 'simple_gplus_avatar_use_extrafield' filter var to TRUE to use an extra field.
 */
function plugin_init() {

	$use_extra_field = apply_filters( 'simple_gplus_avatar_use_extrafield', true );

	if( false !== $use_extra_field ){

		// add the field to comment form
		add_filter(
			'comment_form_defaults',
			__NAMESPACE__ . '\add_comment_field'
		);

		// save data from new comment field on posting a comment
		add_action(
			'comment_post',
			__NAMESPACE__ . '\save_comment_meta_data',
			10,
			1
		);

	}

	// get avatar
	add_filter(
		'get_avatar',
		__NAMESPACE__ . '\get_gplus_avatar',
		10,
		5
	);

}

/**
 * Remove all comment-meta (GPlus profile URLs)
 */
function uninstall() {

	global $wpdb;

	$sql = "DELETE FROM {$wpdb->commentmeta} WHERE meta_key = %s;";

	$result = $wpdb->query( $wpdb->prepare( $sql, get_metakey() ) );

}

/**
 * Get the metakey
 *
 * @return string The used metakey
 */
function get_metakey(){

	return 'SGPA_gplus_profile_url';

}

/**
 * Add an extra field to the comment form
 *
 * @uses apply_filters() 'simple_gplus_avatar_labeltext' Filter the label text for the extra field
 * @param array $default_fields The default comment fields
 * @return arary $default_fields Modified array with extra comment field
 */
function add_comment_field( $default_fields ) {

	$metakey = get_metakey();
	$label_text = apply_filters( 'simple_gplus_avatar_labeltext', 'Google+ Profile URL' );

	$comment_field_template =
	'<p class="comment-form-author">
		<label for="%label%">%label_text%</label>
		<input id="%label%" name="%label%" size="30" type="text" />
	</p>';

	$comment_field_template = str_replace( '%label%', $metakey, $comment_field_template );
	$comment_field_template = str_replace( '%label_text%', $label_text, $comment_field_template );

	$default_fields['fields'][$metakey] = $comment_field_template;

	return $default_fields;

}

/**
 * Save the data from extra comment field
 *
 * @param integer $comment_id ID of the current comment
 */
function save_comment_meta_data( $comment_id ) {

	$metakey = get_metakey();

	add_comment_meta(
		$comment_id,
		$metakey,
		filter_input( INPUT_POST, $metakey, FILTER_SANITIZE_URL ),
		false
	);

}

/**
 * Get the Facebook avatar if an url to the user's profile is set. Else return the avatar created by WordPress
 *
 * @uses apply_filters 'simple_gplus_avatar_apikey' !!!IMPORTANT!!! Setup the api-key with apply_filters
 * @param string $avatar HTML of the avatar image
 * @param int|string|object $id_or_email User ID or user email
 * @param int $size Size of the avatar
 * @param string $default URL to a default image
 * @param string $alt Alternative text to use in image tag. Defaults to 'Facebook Avatar'
 * @return string $fb_avatar <img> tag with avatar
 */
function get_gplus_avatar( $avatar, $id_or_email, $size, $default, $alt ) {

	global $comment;

	$apikey = apply_filters( 'simple_gplus_avatar_apikey', false );

	// no key, no G+ avatar!!
	if( empty( $apikey ) )
		return $avatar;

	$gp_avatar = '';

	// prevent error message on dashboard if the comment ID is not set
	// do NOT use get_comment_ID(), this will raise the error messages again!
	$comment_id = ( isset( $comment->comment_ID ) ) ? $comment->comment_ID : 0;

	$gp_profile_url = get_comment_meta( $comment_id, get_metakey(), true );

	$gp_avatar = ( ! empty( $gp_profile_url ) ) ?
		convert_url_to_gp_avatar( $gp_profile_url, $apikey, $alt, $size ) :
		convert_url_to_gp_avatar( $comment->comment_author_url, $apikey, $alt, $size );

	// reset to default avatar if faild getting avatar from gplus
	return ( empty( $gp_avatar ) ) ?
		$avatar : $gp_avatar;

}

/**
 * Try to convert an url into an Facebook avatar image
 *
 * @uses apply_filters() 'simple_gplus_avatar_alttext' Filter the alternate text for the <img>-tag
 * @param string $url URL to be converted
 * @param string $alt Alternate text for <img>-tag
 * @param integer $size Width & height of the avatar image
 * @return string HTML <img>-tag if succed, else an empty string
 */
function convert_url_to_gp_avatar( $url, $apikey, $alt, $size ) {

	// no key, no data!
	if( empty( $apikey ) )
		return false;

	// predefine some vars
	$gp_avatar_template	= 'https://www.googleapis.com/plus/v1/people/%s/';
	$gp_user			= '';
	$gp_avatar			= '';
	$gp_avatar_url		= '';

	$size = ( is_numeric( $size ) ) ?
		$size : 48;

	$safe_alt = ( false === $alt) ?
	    esc_attr( apply_filters( 'simple_gplus_avatar_alttext', 'Google+ Avatar' ) ) :
	    esc_attr( $alt );

	// grab a number with minimum length of 5 digits
	preg_match( '#/(\d{5,})/#Uuis', $url, $matches );

	if( isset( $matches[1] ) && ! empty( $matches[1] ) )
		$userid = $matches[1];

	if( ! empty( $userid ) ){

		// insert user-id into G+ api-url
		$gp_avatar_url = sprintf( $gp_avatar_template , $userid );

		// add the api key as url-parameter
		$gp_avatar_url = add_query_arg( array( 'key' => $apikey ), $gp_avatar_url );

		// get data from G+ api
		$gp_avatar_url = get_gplus_avatar_from_json( $gp_avatar_url );

		if( ! empty( $gp_avatar_url ) ){

			//set size at end of gp_avatar_url to $size
			$gp_avatar_url = preg_replace( '#(sz)=(\d+)$#Uuis', "$1={$size}", $gp_avatar_url );

			$gp_avatar = "<img alt='{$safe_alt}' src='{$gp_avatar_url}' class='avatar avatar-{$size} photo' height='{$size}' width='{$size}' />";

		}

	}

	return $gp_avatar;

}

/**
 * Get the json data from Google+
 *
 * @uses wp_remote_get()
 * @param string $url URL to the G+ profile
 * @return string $avatar_url URL to the G+ avatar image
 */
function get_gplus_avatar_from_json( $url ) {

	$http_args  = array( 'sslverify' => false );
	$avatar_url = '';
	$response   = wp_remote_get( $url, $http_args );

	if( ! is_wp_error( $response ) ) {

		$data = json_decode( $response['body'] );

		if( isset( $data->image->url ) )
			$avatar_url = $data->image->url;

	}

	return $avatar_url;

}