PHP include, include_once, require and require_once are all different ways to make a file load within another, they each do roughly the same thing but what are the differences and use scenarios?

Use include()for when if the file isn’t found or loaded the page will still function or load without it.

Use include_once()if you don’t want errors to make this file be loaded twice and slow down requests. Or if it has functions that may be re declared twice.

require() is for when you must have this file, stop loading if the file is not present or has errors. Use this for important variables like DB connectors.

require_once()is used for one time loading like functions that cannot be re declared. Do note that this is slower than just require().