flat2tree-2.php

<?php

$comments = [
    1 => ['content' => "The first comment", 'parent' => 0],
    2 => ['content' => "The second comment", 'parent' => 0],
    3 => ['content' => "Reply of first comment", 'parent' => 1],
    4 => ['content' => "Another reply of first comemnt", 'parent' => 1],
    5 => ['content' => "Reply of a reply of the first comment", 'parent' => 4]
];


function mapTree($dataset) {
 $tree = array();
 foreach ($dataset as $id=>&$node) {
  if ($node['parent'] === null) { // root node
    $tree[$id] = &$node;
  } else { // sub node
    $dataset[$node['parent']]['children'][$id] = &$node;
  }
 }
 return $tree;
}

var_dump(mapTree($comments));