It took me some time so I would like to share with you: How-to download a file with PHP's cURL and afterwards continue with other cURL requests which return their output as usual.

php-curl-download-and-revert-curlopt_file.php

$fh = fopen('/your/target/file', 'w');
curl_setopt($ch, CURLOPT_FILE, $fh);
curl_setopt($ch, CURLOPT_HTTPGET, true);
curl_setopt($ch, CURLOPT_URL, 'http://your/download/url');
curl_exec($ch);

// revert it
curl_setopt($this->ch, CURLOPT_FILE, STDOUT);
fclose($fh); // don't forget to close the handle

// important if you want to get the output of the next requests
curl_setopt($this->ch, CURLOPT_RETURNTRANSFER, true);