Merge pull request #7897 from stanley-cheung/php-fix-per-rpc-creds

package.xml

src/php/ext/grpc/call.c

src/php/ext/grpc/call_credentials.c

src/php/tests/interop/interop_client.php

src/php/tests/unit_tests/CallCredentials2Test.php

src/php/tests/unit_tests/CallCredentials3Test.php

src/php/tests/unit_tests/CallTest.php

templates/package.xml.template