How can PHP developers ensure the secure transmission of user credentials and cookies between different servers in a multi-server setup?

To ensure the secure transmission of user credentials and cookies between different servers in a multi-server setup, PHP developers can use HTTPS for communication between servers, encrypt sensitive data before transmission, and implement secure cookie settings such as setting the 'secure' and 'httpOnly' flags.

// Set secure cookie with HTTPOnly flag
setcookie("user", $username, time() + 3600, "/", "example.com", true, true);