<?php
$shop_categories = [
    [ 'title' => 'Компьютеры',
        'children' => [
            [ 'title' => 'Ноутбуки', ],
            [ 'title' => 'Моноблоки', ],
            [ 'title' => 'Системные блоки',
                'children' => [
                    [ 'title' => 'Tower', ],
                    [ 'title' => 'Mini Tower', ]
                ]
            ]
        ]
    ],
    [ 'title' => 'Бытовая техника',
        'children' => [
            [ 'title' => 'Пылесосы', ],
            [ 'title' => 'Холодильники', ]
        ]
    ]
];
function write_title($title, $level) {
    for($i = 0; $i< $level; ++$i) {
        echo '--';
    }
    echo $title.PHP_EOL;
}
function print_tree(array $data, $level = 0) {
    foreach($data as $node) {
        write_title($node['title'], $level);
        if(array_key_exists('children', $node)) {
            print_tree($node['children'], $level + 1);
        }
    }
}

print_tree($shop_categories, 0);