<?php

$entries = [
    1 => [
        'value' => 'entry 1',
    ],
    2 => [
        'value' => 'entry 2',
    ],
];

$subEntries = [
    'a' => [
        'foo' => 'barA',
        'parent_entry' => 1,
    ],
    'b' => [
        'foo' => 'barB',
        'parent_entry' => 1,
    ],
    'c' => [
        'foo' => 'barC',
        'parent_entry' => 2,
    ],
];


$subEntriesPerParentEntry = [];
foreach ($subEntries as $key => $value) {
    $parentEntryId = $value['parent_entry'];
    $subEntriesPerParentEntry[$parentEntryId][$key] = $value;
}

$tree = [];
foreach ($entries as $key => $value) {
    $value['childs'] = $subEntriesPerParentEntry[$key] ?? [];
    $tree[$key] = $value;
}

echo json_encode($tree, JSON_PRETTY_PRINT);