This is the PHP 5 language runtime engine as an Alpine Linux package. It includes a patch for a NaN bug with musl libc.
See the releases page for the latest download links.
The current installation method for these packages is to pull them in using
wget or curl and install the local file with apk:
apk --no-cache add ca-certificates wget
wget -q -O /etc/apk/keys/sgerrand.rsa.pub https://alpine-pkgs.sgerrand.com/sgerrand.rsa.pub
wget https://github.com/sgerrand/alpine-pkg-php5/releases/download/5.6.36-r1/php5-5.6.36-r1.apk
apk add --no-cache php5-5.6.36-r1.apk