I has published composer package of GridDB PHP Client for Centos 7.6(x64) on Packagist repository:
https://packagist.org/packages/griddb/php-client
Above link has both installation and sample execution information.
You can refer to original Github repository at link:
https://github.com/thanhthanh-yen/griddb_php_client
Source code and .so file, .php file are created from GridDB PHP Client with master branch at :
https://github.com/griddb/php_client