There are several distributions that supports the libvirt-php in it's repositories. You can see the list in here:

Distribution Installer command Link More information
Fedora 16+ yum install php-libvirt List on FedoraProject.org -
altLinux N/A List on Sisyphus.ru Distributed as part of php-virt-control project
Gentoo emerge libvirt-php List on packages.gentoo.org -