Description:
------------
PCRE differentiates "a sub-pattern that matched and is empty" from "a sub-pattern that didn't match". But in PHP's exposed APIs both of these cases produce a sub-pattern key with an empty match.

Probably this can be a flag, for unfortunate B.C. reasons, but the current behavior is certainly not correct.

Test script:
---------------
$re = '%^
    ((?=x))?
    ((?=y))?
    (\w)
$%mx';

$str = 'x
y
z';

preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);

// Print the entire match result
var_dump($matches);

Expected result:
----------------
Observe the identical PCRE regex and input on Regex101, notice the flag "isParticipating" clearly points out which pattern matched (even if empty) or didn't match:

[
  [
    {
      "content": "x",
      "isParticipating": true,
      "groupNum": 0,
    },
    {
      "content": "",
      "isParticipating": true,
      "groupNum": 1,
    },
    {
      "content": "",
      "isParticipating": false,
      "groupNum": 2,
    },
    {
      "content": "x",
      "isParticipating": true,
      "groupNum": 3,
    }
  ],
  [
    {
      "content": "y",
      "isParticipating": true,
      "groupNum": 0,
    },
    {
      "content": "",
      "isParticipating": false,
      "groupNum": 1,
    },
    {
      "content": "",
      "isParticipating": true,
      "groupNum": 2,
    },
    {
      "content": "y",
      "isParticipating": true,
      "groupNum": 3,
    }
  ],
  [
    {
      "content": "z",
      "isParticipating": true,
      "groupNum": 0,
    },
    {
      "content": "",
      "isParticipating": false,
      "groupNum": 1,
    },
    {
      "content": "",
      "isParticipating": false,
      "groupNum": 2,
    },
    {
      "content": "z",
      "isParticipating": true,
      "groupNum": 3,
    }
  ]
]




Source: https://regex101.com/r/N90TJE/1

Actual result:
--------------
Both matching-and-empty and non-matching are just empty strings, we can't tell them apart:

array(3) {
  [0]=>
  array(4) {
    [0]=>
    string(1) "x"
    [1]=>
    string(0) ""
    [2]=>
    string(0) ""
    [3]=>
    string(1) "x"
  }
  [1]=>
  array(4) {
    [0]=>
    string(1) "y"
    [1]=>
    string(0) ""
    [2]=>
    string(0) ""
    [3]=>
    string(1) "y"
  }
  [2]=>
  array(4) {
    [0]=>
    string(1) "z"
    [1]=>
    string(0) ""
    [2]=>
    string(0) ""
    [3]=>
    string(1) "z"
  }
}