<?php

    switch ( $default ) {
        case 'mystery':
            $default = "$host/avatar/ad516503a11cd5ca435acc9bb6523536?s={$size}"; // ad516503a11cd5ca435acc9bb6523536 == md5('unknown@gravatar.com')
            break;

        case 'blank':
            if ( empty( $email ) {
                $default = includes_url( 'images/blank.gif' );
            }
            break;

        case 'gravatar_default':
            $default = '';
            break;

        default:
            if ( empty( $email ) {
                $default = "$host/avatar/?d=$default&amp;s={$size}";
            }
            elseif ( strpos( $default, 'http://' ) === 0 )
                $default = add_query_arg( 's', $size, $default );
            }
    }