Difference between require, include, require_once and include_once?

In PHP:

• When should I use require vs. include?
• When should I use require_once vs. include_once?

There are require and include_once as well.

So your question should be...

1. When should I use require vs. include?
2. When should I use require_once vs. require

The answer to 1 is described here.

The require() function is identical to include(), except that it handles errors differently. If an error occurs, the include() function generates a warning, but the script will continue execution. The require() generates a fatal error, and the script will stop.

The answer to 2 can be found here.

The require_once() statement is identical to require() except PHP will check if the file has already been included, and if so, not include (require) it again.

Note that nowadays there is little use for *_once variants. If you are using them, it's likely either a mindless habit or your code needs to be restructured.