#!/usr/bin/env php
<?php

$array = [
    1 => 'a.b.c',
    2 => 'a.b.d',
    3 => 'b.c.e',
    4 => 'a.e',
    5 => 'a',
    6 => 'c',
    7 => 'a.c.e.f',
    8 => 'd.f.g',
    9 => 'b.c.e.f',
    10 => 'a.b.c.d.f'
];

$finalArray = [];

foreach ($array as $id => $entry) {
    $cats = explode('.', $entry);
    $pointer = $finalArray;
    $cats = array_reverse($cats);
    $newValue = [$id];
    foreach ($cats as $cat) {
        $newValue = [$cat => $newValue];
    }
    $finalArray = array_merge_recursive($finalArray, $newValue);
}
ini_set('xdebug.var_display_max_depth', '7');
var_dump($finalArray);