I want to connect Cloud SQL from PHP application on GCE instances.
The PHP application cannot connect to Cloud SQL via SSL connection due to old library version.
I'm considering to connect from GCE instance to Cloud SQL instance without SSL. In that case, is data protected from others?
It is OK that exchanged data is accessible inside GCP network. I just want to prevent data leaked to the Internet and acquired by cracker or stranger.
No, the networking route from GCE to Cloud SQL should not be considered trusted. You should work towards upgrading your PHP application.