Here’s a list of what I’m using at the moment.
apk add --no-cache \
apache2 php7-apache2 php7-pdo \
php7-gd php7-pdo_mysql php7-curl php7-mysqli \
php7-iconv php7-json php7-xml php7-openssl \
php7-sockets php7-ftp php7-session \
php7-xml php7-ctype php7-dom php7-tokenizer
Contra what Symfony says, Alpine’s default installation of PHP does not include all the necessary extensions.