Copy first level keys of a 2d array as a new column of each row

Question

Assuming a 2d array as input, how can each first level key be copied as a new element in its second level row?

Sample input:

$members = [
    'myname' => ['userid' => 52, 'age' => 46],
    'isname' => ['userid' => 22, 'age' => 47],
    'yourname' => ['userid' => 47, 'age' => 85]
];

Desired result:

[
    'myname' => ['userid' => 52, 'age' => 46, 0 => 'myname'],
    'isname' => ['userid' => 22, 'age' => 47, 0 => 'isname'],
    'yourname' => ['userid' => 47, 'age' => 85, 0 => 'yourname'],
]

After fixing some typos, the following script works, but how else can this be achieved?

foreach ($members as $key => $item) {
  array_push($members[$key], $key);
}

_{This question was stimulated by array_push not working in foreach loop which suffered from multiple typos and was closed accordingly.}

Answer 1

• A body-less foreach() loop signature can ignore all pre-existing row elements and solely create a reference to the new column and since the key variable is accessed after the value variable, the $key value is assigned to the referenced column. In my opinion, this is the most elegant, non-functional-style script, but it might not be the most intuitive solution for all developers. Demo

  foreach ($members as $col => [&$col]);
  var_export($members);
  

  The 0 column/key is implied by omitting 0 => before the reference declaration inside of the square braced expression. If you want to give the new column a key other than 0, then write that in the array destructuring expression. Demo

---

• The above snippet is more elegant than declaring the temporary row variable which doesn't need to be used to affect the original array. Demo

  foreach ($members as $col => $row) {
       $members[$col][] = $col;
  }
  var_export($members);
  

---

• Using array_walk() serves the same purpose as a foreach() with its ability to access rows and first level keys, but it doesn't return a payload, so modifying data by reference is the only way. Demo

  array_walk($members, fn(&$row, $col) => $row += [$col]);
  var_export($members);
  

---

• Probably least attractive versus earlier snippets, you could use a functional-style with array_reduce() to append the new column to each row. Demo

  var_export(
       array_reduce(
           array_keys($members),
           fn($result, $k) => $result + [$k => $members[$k] + [$k]],
           []
       )
  );