PHP Proto3: update tests and README

composer.json

src/php/.gitignore

src/php/README.md

src/php/bin/determine_extension_dir.sh

src/php/bin/generate_proto_php.sh

src/php/bin/interop_client.sh

src/php/composer.json

src/php/tests/generated_code/AbstractGeneratedCodeTest.php

src/php/tests/generated_code/math.proto

src/php/tests/interop/empty.proto

src/php/tests/interop/interop_client.php

src/php/tests/interop/messages.proto

src/php/tests/interop/test.proto

templates/composer.json.template

templates/src/php/composer.json.template

.../dockerfile/interoptest/grpc_interop_php/build_interop.sh

...dockerfile/interoptest/grpc_interop_php7/build_interop.sh

...ress_test/grpc_interop_stress_php/build_interop_stress.sh