Includes and requires

Use

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

references

Apply knowledge or rules to put theory into practice.