<?php
    $servername = "localhost";
    $username = "root";
    $password = "";
    $dbname = "databasename";
    // Create connection
    $conn = new mysqli($servername, $username, $password, $dbname);
    // Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }
    $index = array();
    $sql = "SELECT firstname, lastname, ID, parentID FROM table";
    $result = $conn->query($sql);

    while($row = $result->fetch_array()){
    $rows[] = $row;
    }

    // create an index on id
    foreach($rows as $row){
      $index[$row['ID']] = $row;
    }

    // build the tree
    foreach($index as $id => &$row){
      if ($id === 0) continue;
      $parent = $row['parentID'];
      $index[$parent]['children'][] = &$row;
    }
    unset($row);

    // obtain root node
    $index = $index[0]['children'];

    /* free result set */
    $result->close();
    /* close connection */
    $conn->close();

    // output json
    echo json_encode($index, JSON_PRETTY_PRINT);

[
  {
    "0": "Luka",
    "firstname": "Luka",
    "1": "Some",
    "lastname": "Some",
    "2": "287",
    "ID": "287",
    "3": "277",
    "parentID": "277"
},
{
    "0": "John",
    "firstname": "John",
    "1": "Test",
    "lastname": "Test",
    "2": "4080",
    "ID": "4080",
    "3": "277",
    "parentID": "277"
  }
]