src/Mail/mimePart.php
function &addSubpart($body, $params)
    {
        $this->_subparts[] = $part = new Mail_mimePart($body, $params);
        return $part;
    }
function &addSubpart($body, $params)
    {
        $this->_subparts[] = $part = new Mail_mimePart($body, $params);
        return $part;
    }
Adds a subpart to current mime part and returns
a reference to it

@param string $body   The body of the subpart, if any.
@param array  $params The parameters for the subpart, same
as the $params argument for constructor.

@return Mail_mimePart A reference to the part you just added. In PHP4, it is
crucial if using multipart/* in your subparts that
you use =& in your script when calling this function,
otherwise you will not be able to add further subparts.
@access public