In programming, predefined variables act as ready-to-use containers that already contain useful data. You can use them without having to create or fill them yourself because they are predetermined by the system or programming language.
Example using $_SERVER["REMOTE_ADDR"] and $_SERVER["HTTP_USER_AGENT"]:
Your IP Address: 18.221.19.26
Your Browser: Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; ClaudeBot/1.0; +claudebot@anthropic.com)