Simple Facebook Avatar see http://wordpress.stackexchange.com/questions/77851/custom-avatars-in-wordpress-comments

index.php

<?php
/**
 * WordPress-Plugin Simple Facebook Avatar
 *
 * PHP version 5.3
 *
 * @category   PHP
 * @package    WordPress
 * @subpackage Simple Facebook Avatar
 * @author     Ralf Albert <me@neun12.de>
 * @license    GPLv3 http://www.gnu.org/licenses/gpl-3.0.txt
 * @version    0.2.20130101-01
 * @link       http://wordpress.com
 */

/**
 * Plugin Name:	Simple Facebook Avatar
 * Plugin URI:	http://yoda.neun12.de
 * Description:	Replacing the standard avatar in comments with a facebook avatar if a user's facebook profile url is provided
 * Version: 	0.2.20130101-01
 * 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
 *
 *  - Use an extra field
 *  	add_filter( 'simple_facebook_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_facebook_avatar_labeltext', __( '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_facebook_avatar_alttext', $default_alternate_text )
 *  	  Setup an default alternate text for images
 *  	  Default is: Facebook Avatar
 */



/**
 * Main namespace for Simple Facebook Avatar
 * @author Ralf Albert
 *
 */
namespace SimpleFacebookAvatar;

/**
 * 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_facebook_avatar_use_extrafield' filter var to TRUE to use an extra field.
 */
function plugin_init() {

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

	/*
	 * Add comment field
	 * @see http://wpengineer.com/2214/adding-input-fields-to-the-comment-form/
	 *
	 * Get Facebook Avatar
	 * @see http://stackoverflow.com/questions/2821061/facebook-api-how-do-i-get-a-facebook-users-profile-image-through-the-fb-api
	 */

	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_facebook_avatar',
		10,
		5
	);

}

/**
 * Remove all comment-meta (Facebook 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 'SFA_facebook_profile_url';

}

/**
 * Add an extra field to the comment form
 *
 * @uses apply_filters() 'simple_facebook_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_facebook_avatar_labeltext', 'Facebook 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
 *
 * @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_facebook_avatar( $avatar, $id_or_email, $size, $default, $alt ) {

	global $comment;

	$fb_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;

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

	$fb_avatar = ( ! empty( $fb_profile_url ) ) ?
		convert_url_to_fb_avatar( $fb_profile_url, $alt, $size ) :
		convert_url_to_fb_avatar( $comment->comment_author_url, $alt, $size );

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

}

/**
 * Try to convert an url into an Facebook avatar image
 *
 * @uses apply_filters() 'simple_facebook_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_fb_avatar( $url, $alt, $size ) {

	// predefine some vars
	$fb_avatar_template = 'http://graph.facebook.com/%username%/picture';
	$fb_user            = '';
	$fb_avatar          = '';
	$fb_avatar_url      = '';

	$size = ( is_numeric( $size ) ) ?
	    $size : 96;

	$safe_alt = ( false === $alt) ?
	    esc_attr( apply_filters( 'simple_facebook_avatar_alttext', 'Facebook Avatar' ) ) :
	    esc_attr( $alt );

	$parsed_url = parse_url( $url );

	if( isset( $parsed_url['path'] ) ) {

		$fb_user = trim( $parsed_url['path'], '/' );

		if( ! empty( $fb_user ) ){

			$fb_avatar_url = str_replace( '%username%', $fb_user, $fb_avatar_template );
			$fb_avatar = "<img alt='{$safe_alt}' src='{$fb_avatar_url}' class='avatar avatar-{$size} photo' height='{$size}' width='{$size}' />";

		}
	}

	return $fb_avatar;

}

Hello Ralf,
I'm new in WP. I read your plugin... and I'm wondering how to pass user id to your plugin?
am I supposed to send it using get_facebook_avatar function?

In my website, I have used facebook SDK PHP to get user information (including email and id). Now how your plugin will come to action?

Sorry if my question seems stupid. I'm new in WP as stated in the beginning...

Thanks a million
Riad
riad.ericsson@gmail.com

Never mind Ralf,
I've studied the filters in WP and got the point of your code above... it's not what I'm looking for.
However, I'm bit surprised! are you expecting that the normal website visitor would know his facebook id and he would simply put in comment section?