What is the significance of the PORT command successful warning in the PHP script provided?

The PORT command successful warning in the PHP script indicates that the FTP connection is being established using an active mode instead of the passive mode. This can lead to connection issues, especially if the server is behind a firewall. To solve this issue, the FTP connection should be set to use passive mode instead of active mode.

// Set FTP connection to use passive mode
ftp_pasv($ftp_conn, true);