php: make channel

src/php/ext/grpc/channel.c

src/php/ext/grpc/channel_credentials.c

src/php/ext/grpc/package.xml

src/php/lib/Grpc/AbstractCall.php

src/php/lib/Grpc/BaseStub.php

src/php/tests/generated_code/AbstractGeneratedCodeTest.php

src/php/tests/generated_code/GeneratedCodeTest.php

...hp/tests/generated_code/GeneratedCodeWithCallbackTest.php

src/php/tests/interop/interop_client.php