create array tree from array list

One more rework of Thunderstriker's variant - all the logic in one function:

/**
 * @param array $flatList - a flat list of tree nodes; a node is an array with keys: id, parentID, name.
 */
function buildTree(array $flatList)
{
    $grouped = [];
    foreach ($flatList as $node){
        $grouped[$node['parentID']][] = $node;
    }

    $fnBuilder = function($siblings) use (&$fnBuilder, $grouped) {
        foreach ($siblings as $k => $sibling) {
            $id = $sibling['id'];
            if(isset($grouped[$id])) {
                $sibling['children'] = $fnBuilder($grouped[$id]);
            }
            $siblings[$k] = $sibling;
        }
        return $siblings;
    };

    return $fnBuilder($grouped[0]);
}

// Example:
$flat = [
    ['id' => 100, 'parentID' => 0, 'name' => 'root'],
    ['id' => 101, 'parentID' => 100, 'name' => 'ch-1'],
    ['id' => 102, 'parentID' => 101, 'name' => 'ch-1-1'],
    ['id' => 103, 'parentID' => 101, 'name' => 'ch-1-2'],
];

$tree = buildTree($flat, 'parentID', 'id');
echo json_encode($tree, JSON_PRETTY_PRINT);

Playground: https://www.tehplayground.com/Ap4uUuwHWl9eiJIx

small fix if you need more than 1 parentid[0] element :)

$arr = array(
  array('id'=>100, 'parentid'=>0, 'name'=>'a'),
  array('id'=>101, 'parentid'=>100, 'name'=>'a'),
  array('id'=>102, 'parentid'=>101, 'name'=>'a'),
  array('id'=>103, 'parentid'=>101, 'name'=>'a'),
);

$new = array();
foreach ($arr as $a){
    $new[$a['parentid']][] = $a;
}
$tree = createTree($new, $new[0]); // changed
print_r($tree);

function createTree(&$list, $parent){
    $tree = array();
    foreach ($parent as $k=>$l){
        if(isset($list[$l['id']])){
            $l['children'] = createTree($list, $list[$l['id']]);
        }
        $tree[] = $l;
    } 
    return $tree;
}

oke this is how i solved it:

$arr = array(
  array('id'=>100, 'parentid'=>0, 'name'=>'a'),
  array('id'=>101, 'parentid'=>100, 'name'=>'a'),
  array('id'=>102, 'parentid'=>101, 'name'=>'a'),
  array('id'=>103, 'parentid'=>101, 'name'=>'a'),
);

$new = array();
foreach ($arr as $a){
    $new[$a['parentid']][] = $a;
}
$tree = createTree($new, array($arr[0]));
print_r($tree);

function createTree(&$list, $parent){
    $tree = array();
    foreach ($parent as $k=>$l){
        if(isset($list[$l['id']])){
            $l['children'] = createTree($list, $list[$l['id']]);
        }
        $tree[] = $l;
    } 
    return $tree;
}