For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mmkiwi/openSUSE_Tumbleweed/home:mmkiwi.repo
zypper refresh
zypper install php8
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mmkiwi/15.5/home:mmkiwi.repo
zypper refresh
zypper install php8
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mmkiwi/15.4/home:mmkiwi.repo
zypper refresh
zypper install php8