PHP ftp_pasv() Function

<?php
// connect and login to FTP server
$ftp_server = "ftp.example.com";
$ftp_conn = ftp_connect($ftp_server) or die("Could not connect to $ftp_server");
$login = ftp_login($ftp_conn, $ftp_username, $ftp_userpass);

// turn passive mode on
ftp_pasv($ftp_conn, true);

$file = "localfile.txt";

// upload file
if (ftp_put($ftp_conn, "serverfile.txt" , $file, FTP_ASCII))
  {
  echo "Successfully uploaded $file.";
  }
else
  {
  echo "Error uploading $file.";
  }

// close connection
ftp_close($ftp_conn);
?>