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;
	}
}

Why use wp_cache instead of wp_transients ?

By default, the object cache is non-persistent. This means that data stored in the cache resides in memory only and only for the duration of the request. Cached data will not be stored persistently across page loads unless you install a persistent caching plugin.

Well done, BRAVO!

If anyone has stumbled across this and has found it isn't working, you'll want to update http:// to https:// for the Gravatar link, as it now only supports SSL.

See here: Automattic/themes#7515