framework/filters/HttpCache.php
protected function generateEtag($seed)
    {
        $etag = '"' . rtrim(base64_encode(sha1($seed, true)), '=') . '"';
        return $this->weakEtag ? 'W/' . $etag : $etag;
    }
protected function generateEtag($seed)
    {
        $etag = '"' . rtrim(base64_encode(sha1($seed, true)), '=') . '"';
        return $this->weakEtag ? 'W/' . $etag : $etag;
    }
Generates an ETag from the given seed string.
@param string $seed Seed for the ETag
@return string the generated ETag