debian-php-backports

apt repository with PHP security fixes for archived Debian releases (and Ubuntu 14.04 trusty, 16.04 xenial, 18.04 bionic, 20.04 focal).

curl -fsSL https://animaartificialis.github.io/debian-php-backports/debian-php-backports.asc \
  | gpg --dearmor -o /usr/share/keyrings/debian-php-backports.gpg
echo "deb [signed-by=/usr/share/keyrings/debian-php-backports.gpg] https://animaartificialis.github.io/debian-php-backports buster sury" \
  > /etc/apt/sources.list.d/debian-php-backports.list

Suites, components and older releases are described in the README.
Debian 9 stretch and 10 buster with their own PHP (7.0, 7.3): use Freexian ELTS instead. Signing key: debian-php-backports.asc.