When you are using CURLOPT_FILE to download directly into a file you must close the file handler after the curl_close() otherwise the file will be incomplete and you will not be able to use it until the end of the execution of the php process.
<?php
$fh = fopen('/tmp/foo', 'w');
$ch = curl_init('http://example.com/foo');
curl_setopt($ch, CURLOPT_FILE, $fh);
curl_exec($ch);
curl_close($ch);
# at this point your file is not complete and corrupted
fclose($fh);
# now you can use your file;
read_file('/tmp/foo');
?>