What is the difference between PHP require and include?

Question

I know the basic usage of PHP require, require once, include and include once. But I am confused about when I should use them.

Example: I have 3 files, eg: settings.

Answer 1

• require
  when the file is required by your application, e.g. an important message template or a file containing configuration variables without which the app would break.

• require_once
  when the file contains content that would produce an error on subsequent inclusion, e.g. function important() { /* important code */} is definitely needed in your application but since functions cannot be redeclared should not be included again.

• include when the file is not required and application flow should continue when not found, e.g.
  great for templates referencing variables from the current scope or something

• include_once
  optional dependencies that would produce errors on subsequent loading or maybe remote file inclusion that you do not want to happen twice due to the HTTP overhead