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.
It is possible to insert the content of one PHP file into another PHP file (before the server executes it), with the include or require statement. The include and require statements are identical, except upon failure: require will produce a fatal error (E_COMPILE_ERROR) and stop the script.
Although this OP has posted a valid, yet pointless question in this forum. A similarly pointless question has appeared elsewhere. This question has been asked many times before and this has been answered adequately by cereal here. So let's leave it there.