protected function getBoundary()
    {
        do {
            $boundary = '==' . md5( microtime() ) . '==';
        } while ( in_array( $boundary, self::$boundaries ) );

        return self::$boundaries[] = $boundary;
    }
protected function getBoundary()
    {
        do {
            $boundary = '==' . md5( microtime() ) . '==';
        } while ( in_array( $boundary, self::$boundaries ) );

        return self::$boundaries[] = $boundary;
    }
protected function getBoundary ( ) { do { $ boundary = '==' . md5 ( microtime ( ) ) . '==' ; } while ( in_array ( $ boundary , self :: $ boundaries ) ) ; return self :: $ boundaries [ ] = $ boundary ; }
Get unique boudary

Get a unique boundary string, which is not yet used by any wrapping
document and most probably not occurs in any of the embedded documents.
We cannot be entirely sure about that, but do not want waiting to define
the boundary until we loaded all documents into memory.

@return string
Get unique boudary