PHP ftp_put Fails: Analyzing the Issue and Resolving It

The inability of ftp_put() to function correctly can be a frustrating problem when transferring files over FTP. In PHP, a common cause for this issue lies in the default use of active mode.

Active vs. Passive Mode Transfer

Active mode dictates that the FTP server connects to the client on a specified port. Passive mode, on the other hand, has the server listen on a random port, which is then provided to the client. In most cases, using passive mode is necessary, especially when the FTP server is behind a firewall or NAT.

Resolving the Issue

To switch PHP's FTP transfer to passive mode, one must invoke the ftp_pasv() function:

$connect = ftp_connect($ftp) or die("Unable to connect to host");
ftp_login($connect, $username, $pwd) or die("Authorization failed");

// Switch to passive mode
ftp_pasv($connect, true) or die("Unable switch to passive mode");

Additional Considerations

In some cases, the FTP server may report an incorrect IP address in response to the PASV command. This can be addressed by disabling the use of the PASV address:

ftp_set_option($connect, FTP_USEPASVADDRESS, false);

Best Practices

While the above solutions can alleviate the problem, the optimal solution is to configure the FTP server to use the correct IP address and port configuration. This ensures proper operation in all scenarios.