Support PHP7

[田欧]

I have forked code the grpc php extension, and upgraded grpc PHP extensions from PHP5 to PHP7. It may be have some problems, but can right run example and unittest. links:https://github.com/thinkerou/grpc-php7

If the directory src/php/ext/ has two directories which are grpc and grpc-php7, this will need to maintain two codes when the code changed; I want to attempt to use marco to wrap function that can use in php5 and php7, but the method make the code ugly.