Why would you want to include/require multiple times?

Question

require and include have the variations require_once and include_once. They prevent the script from loading a file multiple times.

I think I can safely assume, because the functions exist, that there would be cases where you would need the require/include function instead of the require_once/include_once one. But I cannot imagine a case like that. What would that be?

Answer 1

Probably the best example would be when giving output. If, say, you had a snippet of HTML that might appear more than once on various pages, you could put it into a separate file and include it as many times as you wanted.

Moreover, it's often unnecessary to use require_once or include_once. If the file will only be called once (e.g. in __autoload) then the simple functions have less overhead, because PHP doesn't need to keep track of whether the file has been included before.

Answer 2

One may want to use require_once instead of require if you want to have a variable or constant defined only once. This is helpful for making reusable classes in a very large code base that manages dependencies rapidly. Lets say I have a file A that needs a file B but I already include file C that included file B. Since file B was already included, the program does not have to evaluate that code again to properly evaluate file A. This is a case where you may want to include the file only once.

It helps with managing your dependencies