private function _openFile($filename)
    {
        // Check local tmp directory
        if (!is_dir($this->config['tmp']) || !is_writeable($this->config['tmp'])) {
            $this->fail('tmp directory not found or is not writable');
        }

        // Download file to local tmp directory
        $tmp_file = $this->config['tmp'] . "/ftp_data_file.tmp";

        if ($this->isSFTP()) {
            $downloaded = @$this->ftp->get($filename, $tmp_file);
        } else {
            $downloaded = @ftp_get($this->ftp, $tmp_file, $filename, FTP_BINARY);
        }
        if (!$downloaded) {
            $this->fail('failed to download file to tmp directory');
        }

        // Open file content to variable
        if ($this->file = file_get_contents($tmp_file)) {
            $this->filepath = $filename;
        } else {
            $this->fail('failed to open tmp file');
        }
    }