php-libvirt-php missing functions?

I'm trying to write some PHP scripts using php-libvirt-php. I've been basing it on a script I saw online but I'm getting errors about undefined functions, eg:

Fatal error: Uncaught Error: Call to undefined function libvirt_get_hostname() in /var/www/html/libvirt_test1.php:19 Stack trace: #0 {main} thrown in /var/www/html/libvirt_test1.php on line 19

But libvirt_get_hostname() is supposed to be part of php-libvirt-php, isn't it? The list of functions is at http://phplibvirt.cybersales.cz/doc/book.libvirt.html

So is there something wrong with the package I've installed, or am I misunderstanding something here?