[giesch]

In a procedural language:

x == 3 means: Take the value out of the box x. Is it 3?

x = 3 means: x is a box, put 3 in it. The 3 lasts forever, or until someone changes it.

In a functional language:

x = 3 means: Take the value out of box x. Is it 3?

let x = 3 in [...] means: x is a box, put a 3 in it. The 3 lasts for only the [...], but cannot be undone.

[nybble41]

Correction: In a functional language (specifically Haskell):

  x = 3 means: x is defined as 3.
  x == 3 means: Is x equal to 3?
  let x = 3 in [...] means: within the scope of [...], x is defined as 3.

There is no specific point where something is "put into the box" or "taken out of the box". In fact, there is no "box". The functional programming concept of "bindings" does not correspond to the imperative concept of "variables", not even the oxymoron known was "constant variables". (If you want variables in Haskell you need IORef or STRef, where the load and store operations are explicit monadic actions.)