Pues debes compilar PHP con opción (o buscarte el RPM o paquete de instalación adecuado .. tal vez un php-ftp o similar)
Cita: Installation
In order to use FTP functions with your PHP configuration, you should add the --enable-ftp option when installing PHP 4 or greater or --with-ftp when using PHP 3.
The windows version of PHP has built in support for this extension. You do not need to load any additional extension in order to use these functions.
Extraido de:
www.php.net/ftp
Un saludo,