The gopacket package provides a Go wrapper for libpcap written in C. It is more than just a simple wrapper though. It provides additional functionality and takes advantage of Go things like interfaces, which makes it incredibly powerful.
SSL sockets are perfect for sending secure data. With certificates, you can verify the identify of the host, the client, or both. Signed certificates cost money but you can create and self-sign a certificate. Check out the code samples below to see how to generate SSL certificates and create SSL clients and servers. Examples include raw socket communication as well as the common HTTPS protocol.
With PHP, creating a TCP server takes only a few lines of code. Check out this code that creates a server which responds with the current time.
Unit testing a TCP server and client can be tricky at first. Let me share what I've come up with for unit testing them. I will not provide full test class code just invidual test methods. If you don't know how to use unittest with Python and get your tests running refer to the unittest documentation. GameServer.Server and GameClient.Client are the classes that are being tested and which should be implemented elsewhere.