Do you have OPcache turned on?

If so, it might just have optimised it out. Can you provide a short reproducible script, and a remote log (as is described at https://xdebug.org/support.php#remote) to go with it please?

Short script. Both variables $a and $b did not show while stepping through the code. I still have to check whether OPcache is turned on:

public function test()
{
$a = 1;
$b = $a + 1;
return $b;
}

Just checked: OPcache is turned on.

This is not a bug, but a feature in OPcache, which disables and optimises PHP code.

With OPcache enabled, the following PHP opcodes are created:

compiled vars:  none

line     #* E I O op                           fetch          ext  return  operands

2 0 E > EXT_NOP
4 1 EXT_STMT
5 2 EXT_STMT
6 3 EXT_STMT
4 > RETURN 2

As you can see, it only really does a "return 2;", and has no variables. The EXT_STMT opcodes are the ones that Xdebug can break on.

With OPcache disabled, the ADD and ASSIGN statements, as well as the $a and $b variables are still there:

compiled vars:  !0 = $a, !1 = $b

line     #* E I O op                           fetch          ext  return  operands

2 0 E > EXT_NOP
4 1 EXT_STMT
2 ASSIGN !0, 1
5 3 EXT_STMT
4 ADD 0000003:0000003 !0, 1
5 ASSIGN !1, 0000003:0000003
6 6 EXT_STMT
7 > RETURN !1
7 8 EXT_STMT
9 > RETURN null