In wordpress, a better way to check if an author has a gravatar or not. Sometimes you might want to check to see if a gravatar exists and not display any image if there isn't one. Uses the wordpress HTTP and caching apis. A better version of this: http://codex.wordpress.org/Using_Gravatars#Checking_for_the_Existence_of_a_Gravatar

validate_gravatar.php

/**
 * Utility function to check if a gravatar exists for a given email or id
 * @param int|string|object $id_or_email A user ID,  email address, or comment object
 * @return bool if the gravatar exists or not
 */

function validate_gravatar($id_or_email) {
  //id or email code borrowed from wp-includes/pluggable.php
	$email = '';
	if ( is_numeric($id_or_email) ) {
		$id = (int) $id_or_email;
		$user = get_userdata($id);
		if ( $user )
			$email = $user->user_email;
	} elseif ( is_object($id_or_email) ) {
		// No avatar for pingbacks or trackbacks
		$allowed_comment_types = apply_filters( 'get_avatar_comment_types', array( 'comment' ) );
		if ( ! empty( $id_or_email->comment_type ) && ! in_array( $id_or_email->comment_type, (array) $allowed_comment_types ) )
			return false;

		if ( !empty($id_or_email->user_id) ) {
			$id = (int) $id_or_email->user_id;
			$user = get_userdata($id);
			if ( $user)
				$email = $user->user_email;
		} elseif ( !empty($id_or_email->comment_author_email) ) {
			$email = $id_or_email->comment_author_email;
		}
	} else {
		$email = $id_or_email;
	}

	$hashkey = md5(strtolower(trim($email)));
	$uri = 'http://www.gravatar.com/avatar/' . $hashkey . '?d=404';

	$data = wp_cache_get($hashkey);
	if (false === $data) {
		$response = wp_remote_head($uri);
		if( is_wp_error($response) ) {
			$data = 'not200';
		} else {
			$data = $response['response']['code'];
		}
	    wp_cache_set($hashkey, $data, $group = '', $expire = 60*5);

	}		
	if ($data == '200'){
		return true;
	} else {
		return false;
	}
}