Fatal error: Call to undefined function ftp_connect()

Why this error happens??
How can I fix it?

my code:

$ftp_conn = ftp_connect($ftp_server) or die("Could not connect to $ftp_server");
$login = ftp_login($ftp_conn, $ftp_username, $ftp_userpass);

[filsil]

You can't use ftp to connect from our servers to another, you can only use http or https.

[Gianluca]

ftp-* functions are currently unavailable