Check files on remote FTP server for duplicate content with PHP - php

Question

I've written a script that transfers local files into a folder structure on a remote FTP server with PHP. I'm currently using ftp_connect() to connect to the remote server and ftp_put() to transfer the file, in this case, a CSV file.
My question is, how would one verify that a file's contents (on the remote FTP server) are not a duplicate of the local file's contents? Is there any way to parse the contents of the remote file in question, as well as a local version and then compare them using a PHP function?
I have tried comparing the filesizes of the local file using filesize() and the remote file using ftp_size(), respectively. However, even with different data, but the same number of characters it generates a false positive for duplication as the file-sizes are the same number of bytes.
Please note, the FTP in question is not under my control, so I can't put any scripts on the remote server.
Update
Thanks to both Mark and gafreax, here is the final working code:
$temp_local_file = fopen($conf['absolute_installation_path'] . 'data/temp/ftp_temp.csv', 'w');
if ( ftp_fget($connection, $temp_local_file, $remote_filename, FTP_ASCII) ) {
$temp_local_stream = file_get_contents($conf['absolute_installation_path'] . 'data/temp/ftp_temp.csv');
$local_stream = file_get_contents($conf['absolute_installation_path'] . $local_filename);
$temp_hash = md5($temp_local_stream);
$local_hash = md5($local_stream);
if($temp_hash !== $local_hash) {
$remote_file_duplicate = FALSE;
} else {
$remote_file_duplicate = TRUE;
}
}

Answer 1

You can use hashing function like md5 and check against two generated md5 if they match.
For example:
$a = file_get_contents('a_local');
$b = file_get_contents('b_local');
$a_hash = md5($a);
$b_hash = md5($b);
if($a_hash !== $b_hash)
echo "File differ";
else
echo "File are the same";
The md5 function is useful to avoid problem on reading strange data on file

Answer 2

You could also compare the last modified time of each file. You'd upload the local file only if it is more recent than the remote one. See filemtime and ftp_mdtm. Both of those return a UNIX timestamp you can easily compare. This is faster than getting the file contents and calculating a hash.