In PHP, a require statement will include and evaluate the specified file.
require 'path/to/some/file.php'; A require_once statement does the same but will check if the file has already been included and, if so, not include it again.
require_once 'path/to/some/file.php'; The benefit of using require_once is very clear but what's the advantage of using a require statement instead of require_once?
Ref:
That clearly depends on the contents of the file you're requiring and whether it should only be included once or explicitly again and again every time you request it; think partial templates as an example for where you might load the same file multiple times.
require() includes and evaluates a specific file, while require_once() does that only if it has not been included before (on the same page).
So, require_once() is recommended to use when you want to include a file where you have a lot of functions for example. This way you make sure you don't include the file more times and you will not get the "function re-declared" error.
Some good info related to your question on this thread.
Specifically on the advantage of using require() over require_once(): require() executes ever so slightly faster so it could be beneficial to use if you're sure there are no duplicate requirements
requireis microspeed, i can't imagine where i would userequire