ftp_fput

For directly inserting content into a file on an FTP host, you could also create a string stream wich streams directly to the ftp_fput function. 
This should create less overhead than first writing to any temp directories locally before streaming, as suggested here.
<?php
$string = "Your content goes here";
$stream = fopen('data://text/plain,' . $string,'r');
ftp_fput($this->connection,$pathTo,$stream, FTP_BINARY);
?>

If when using fput you get the one of the following errors:
Warning: ftp_fput() [function.ftp-fput]: Opening ASCII mode data connection
Warning: ftp_fput() [function.ftp-fput]: Opening BINARY mode data connection
and it creates the file in the correct location but is a 0kb file and all FTP commands thereafter fail. It is likely that the client is behind a firewall. To rectify this use:
<?php
ftp_pasv($resource, true);
?>

Before executing any put commands. Took me so long to figure this out I actually cheered when I did :D

When you have your file contents as a string, create temporary stream and use that as a file handle.
<?php
$contents = "This is a test file\nTesting 1,2,3..";
$tempHandle = fopen('php://temp', 'r+');
fwrite($tempHandle, $contents);
rewind($tempHandle); 
ftp_fput($this->ftp, $filename, $tempHandle, FTP_ASCII);
?>

This might be obvious to most of you, but make sure your stream isn't write-only. It has to be able to read from your stream in order to upload its contents.
Took me a while trying to figure out why my uploaded file was 0B, and that was why.

You might also want to note that ftp_fput will overwrite any existing file.

Make sure you chdir to remote directory before using ftp_put or else ftp_put will just return error that it cannot create file. After you do the chdir you should NOT pass the whole path of file to ftp_put but just basename (filename). See example for more info.
Example:
<?php
$locpath = 'local_path/resources/js/test.js';
$rempath = 'resources/js/';
$remFile = 'test.js';
ftp_chdir($this->conn_id, $rempath);
ftp_put($this->conn_id, $remFile, $locpath, FTP_BINARY);
?>

Using jopi paranoid fi's example, tmpfile() works on PHP 4 and 5 instead of using the php://temp file.

Fails if destination file exists. Delete first and it works.

This is a function i wrote to copy a complete directory to a FTP-Server-folder.
function ftp_uploaddirectory($conn_id, $local_dir, $remote_dir)
{
 @ftp_mkdir($conn_id, $remote_dir);
 $handle = opendir($local_dir);
 while (($file = readdir($handle)) !== false)
 {
 if (($file != '.') && ($file != '..'))
 {
 if (is_dir($local_dir.$file))
 {
 ftp_uploaddirectory($conn_id, $local_dir.$file.'/', $remote_dir.$file.'/');
 }
 else
 $f[] = $file;
 }
 }
 closedir($handle);
 if (count($f))
 {
 sort($f);
 @ftp_chdir($conn_id, $remote_dir);
 foreach ($f as $files)
 {
 $from = @fopen("$local_dir$files", 'r');
 @ftp_fput($conn_id, $files, $from, FTP_BINARY);
 }
 }
}
Example:
$conn_id = @ftp_connect($server);
@ftp_login ($conn_id, $username, $passwort);
ftp_uploaddirectory($conn_id, 'mydirectory/', 'theftpdirectory/');
@ftp_quit($conn_id);
I hope you'll find it useful.

If you want to pass a string containing the filename as source and not a resource handle use ftp_put() instead. Trivial but not mentioned here.