PHP: add various scenarios to test PECL extension

examples/php/echo/README.md

examples/php/echo/apache.Dockerfile

examples/php/echo/base.Dockerfile

examples/php/echo/cli.Dockerfile

examples/php/echo/client.php

examples/php/echo/composer.json

examples/php/echo/echo.proto

examples/php/echo/fpm.Dockerfile

examples/php/echo/nginx.conf

src/php/bin/build_all_docker_images.sh

src/php/bin/run_all_docker_images.sh

src/php/docker/README.md

src/php/docker/alpine/Dockerfile

src/php/docker/fork-support/Dockerfile

src/php/docker/fork-support/fork.php

src/php/docker/grpc-ext/Dockerfile

src/php/docker/grpc-src/Dockerfile

src/php/docker/php-future/Dockerfile

src/php/docker/php-src/Dockerfile

src/php/docker/php-zts/Dockerfile