Understanding Incrementing

That's why it's called the "post-incrementing operator". Essentially, everything is an expression which results in a value. a + 1 is an expression which results in the value 124. If you assign this to b with b = a + 1, b has the value of 124. If you do not assign the result to anything, a + 1 will still result in the value 124, it will just be thrown away immediately since you're not "catching" it anywhere.

BTW, even b = a + 1 is an expression which returns 124. The resulting value of an assignment expression is the assigned value. That's why c = b = a + 1 works as you'd expect.

Anyway, the special thing about an expression with ++ and -- is that in addition to returning a value, the ++ operator modifies the variable directly. So what happens when you do b = a++ is, the expression a++ returns the value 123 and increments a. The post incrementor first returns the value, then increments, while the pre incrementor ++a first increments, then returns the value. If you just wrote a++ by itself without assignment, you won't notice the difference. That's how a++ is usually used, as short-hand for a = a + 1.

This is pretty standard.

Note that you can also write

b = ++a;

Which has the effect you are probably expecting.

It's important to realise that there are two things going on here: the assignment and the increment and the language should define in which order they will happen. As we have available both ++a and a++ it makes sense that they should have different meanings.

For those of us from a C background, this is quite natural. If PHP behaves differently, we might be wondering why PHP chose to deviate from what we are accustomed to.

++ can be used as post-increment operator like in your example, or it could be used as a pre-increment operator if used before variable.

var b = ++a;

Then first the variable a will be incremented, then the incremented value is assigned to b.