build-tree-from-list.php

$k = [
    'a/b/c/j',
    'a/b/c',
    'a/b/d/q',
    'a/b/d',
    'a/b/e',
    'a/b/c/j',
    'a/b/c',
    'a/b/d/q',
    'a/b/d',
    'a/b/e',
    'a/b',
    'a',
    'a/x/z',
    'a/x/y',
    'a/x',
];

$cats = array_map(function ($cat) {
    $t = array_reverse(explode('/', $cat));
    $i = array_shift($t);
    return array_reduce($t, function ($carry, $item) {
        return [$item => $carry];
    }, [$i => []]);
}, $k);

print_r(array_merge_recursive(...$cats));