Are PHP zvals mutable?

Question

I've been reading about memory management in PHP and learned that variables in PHP copy the reference to zvals as long as you don't do a write operation (copy on write paradigm).

https://www.phpinternalsbook.com/php5/zvals/memory_management.html#reference-counting-and-copy-on-write

However, it does not describe what happens when you reassign a value to an already copied zval.

Here's what the book says:

$a = 1;    // $a =           zval_1(value=1, refcount=1)
$b = $a;   // $a = $b =      zval_1(value=1, refcount=2)
$c = $b;   // $a = $b = $c = zval_1(value=1, refcount=3)

$a++;      // $b = $c = zval_1(value=1, refcount=2)
           // $a =      zval_2(value=2, refcount=1)

Now, if you do another $a++, will it change the value in zval,

$a++; // $a = zval_2(value=3, refcount=1)

or will it create a new zval once again?

$a++; // $a = zval_3(value=3, refcount=1)

Following the logic of the PHP Language Reference, I guess it should be more likely option #1 as long as refcount = 1 (you would not manipulate another variable).