Answer 1

"require" is identical to "include" except upon failure it will also produce a fatal E_COMPILE_ERROR level error. In other words, it will halt the script whereas "include" only emits a warning (E_WARNING) which allows the script to continue.

The "include_once", same as "require_once" statement includes and evaluates the specified file during the execution of the script.
This is a behavior similar to the include statement, with the only difference being that if the code from a file has already been included, it will not be included again, and include_once returns TRUE.
As the name suggests, the file will be included just once.