Transfer in-memory data to FTP server without using intermediate file

The file_put_contents is the easiest solution:

file_put_contents('ftp://username:pa‌​ssword@hostname/path/to/file', $contents);

If it does not work, it's probably because you do not have URL wrappers enabled in PHP.

If you need greater control over the writing (transfer mode, passive mode, offset, reading limit, etc), use the ftp_fput with a handle to the php://temp (or the php://memory) stream:

$conn_id = ftp_connect('hostname');

ftp_login($conn_id, 'username', 'password');
ftp_pasv($conn_id, true);

$h = fopen('php://temp', 'r+');
fwrite($h, $contents);
rewind($h);

ftp_fput($conn_id, '/path/to/file', $h, FTP_BINARY, 0);

fclose($h);
ftp_close($conn_id);

(add error handling)

Or you can open/create the file directly on the FTP server. That's particularly useful, if the file is large, as you won't have keep whole contents in memory.

See Generate CSV file on an external FTP server in PHP.

According to Can you append lines to a remote file using ftp_put() or something similar? and Stream FTP Upload with PHP? you should be able to do something using either CURL or PHP's FTP wrappers using file_put_contents().

$data = json_encode($object);
file_put_contents("ftp://user:pass@host/dir/file.ext", $data, FILE_APPEND);