What most C derivative programming languages call a "function" is not actually a "function" in the sense meant in "functional programming". It is more accurately called a "procedure". While you can write a "functional function" in php, there's nothing inherent about php's "functions" that makes them "functional". it's up to the programmer to make them so.
On the other hand, a "formula" in excel is inherently a "functional" function or "first order function". Why? two things: No side effects, and statelessness- that is, the order of the computation is theoretically implicit: a value can be computed in a number of different arbitrary orders without effecting the final result.-- not explicit- like in php, where you are specifying an exact order of operations by the order of statements in a procedure ("function").
Another thing about statelessness is that given the same input, a "first order function" must always return the same output.
The fact that order is not explicit in excel, that there is no user controlled "state" means that there's certain associative, commutative and compositional properties available to "first order" functions that are not possible with a procedure that is not guaranteed to be a first order function.
it's the difference between deciding to do functional programming in an imperative language, and having functional programming structurally enforced by the language.
