The difference between $a+$a++ and $a+$b++

Question

What's the difference between these two?
Why their outputs are different?

<?php

$a = 3;
echo $a + $a++; // 7

$c = 3;
$d = 3;
echo $c + $d++; // 6

?>

Answer 1

The evalution. When same using same var, it recognizes that you can.just sum up (3+3)and increment.

But with different vars, the evaluation is made this way

A temp var is created with the size of an int
$ is read, so it expands next variable
c value is given to the temp var
plus signal is read, so next var will be sum with c
$ again
d is found, sum up with c and add to the temp var
increment d

So d isn't returned to the temp var.

Answer 2

Alexander Santos Thank you. It helped.