Services/Libravatar.php
protected function processDefault($url)
    {
        if ($url === null) {
            return $url;
        }

        $url = (string)$url;

        switch ($url) {
        case '404':
        case 'mm':
        case 'identicon':
        case 'monsterid':
        case 'wavatar':
        case 'retro':
            break;
        default:
            $valid = filter_var($url, FILTER_VALIDATE_URL);
            if (!$valid) {
                throw new InvalidArgumentException('Invalid default avatar URL');
            }
            break;
        }

        return $url;
    }
protected function processDefault($url)
    {
        if ($url === null) {
            return $url;
        }

        $url = (string)$url;

        switch ($url) {
        case '404':
        case 'mm':
        case 'identicon':
        case 'monsterid':
        case 'wavatar':
        case 'retro':
            break;
        default:
            $valid = filter_var($url, FILTER_VALIDATE_URL);
            if (!$valid) {
                throw new InvalidArgumentException('Invalid default avatar URL');
            }
            break;
        }

        return $url;
    }
Verify and cast the default URL to use when no avatar image can be found.
If none is set, the libravatar logo is returned.

@param string $url Full URL to use OR one of the following:
- "404" - give a "404 File not found" instead of an image
- "mm"
- "identicon"
- "monsterid"
- "wavatar"
- "retro"

@return string Default URL

@throws InvalidArgumentException When an invalid URL is given