PHP: make tests runnable on both protobuf 3.1 and 3.2

composer.json

src/compiler/php_generator.cc

src/compiler/php_generator.h

src/compiler/php_generator_helpers.h

src/compiler/php_plugin.cc

src/php/composer.json

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

templates/composer.json.template

templates/src/php/composer.json.template