In the official repositories of Debian 12, the php 8.5 package is not available. To install it, you need to add an additional repository.

1. Update packages

apt update
apt upgrade -y
apt install -y lsb-release ca-certificates curl apt-transport-https

2. Add the repository

curl -sSLo /tmp/debsuryorg-archive-keyring.deb https://packages.sury.org/debsuryorg-archive-keyring.deb
dpkg -i /tmp/debsuryorg-archive-keyring.deb
sh -c 'echo "deb [signed-by=/usr/share/keyrings/debsuryorg-archive-keyring.gpg] https://packages.sury.org/php/ $(lsb_release -sc) main" > /etc/apt/sources.list.d/php.list'

3. Update packages and check the availability of php 8.5

apt update
apt-cache policy php8.5

4. Install php 8.5 packages with fpm

apt install -y php8.5-fpm php8.5-cli php8.5-{bcmath,bz2,curl,gd,gmp,intl,mbstring,mysql,readline,xml,zip}

The opcache package is not installed separately, as it is already built in by default.

5. Enable auto-start

systemctl enable php8.5-fpm --now

6. Check the service status

systemctl status php8.5-fpm

If you're using it with nginx, don’t forget to change the settings there.

Done.