What is the difference between include and include_once (or require and require_once) in php?

Question

I don't understand, please give me proper answer with an real life example.

Answer 1

If an error happens, include will give a warning, but the script will still run. Require will give a fatal error and the script will be completely stopped. require_once and include_once are similar with require and include, but they will check whether the file are included or no, so they don't have to include it again