Recursive table self-reference

categories.php

<?php

class MyDb {
    protected $link = null;

    public function __construct() {
        $this->link = mysqli_connect('host', 'username', 'password', 'database');
    }

    public function getCategoryByName($name) {
        $stmt = $this->link->prepare('select * from categories where name like ?');
        $stmt->bind_param('s', $name);
        $stmt->execute();
        $result = $stmt->get_result();
        return $result->fetch_object();
    }

    public function getParent($cat) {
        $stmt = $this->link->prepare('select * from categories where category_id like ?');
        $stmt->bind_param('i', $cat->parent_id);
        $stmt->execute();
        $result = $stmt->get_result();

        $parent = $result->fetch_object();
        if ($parent->parent_id != 0) {
            return $this->getParent($parent);
        }

        return $parent;
    }
}

$db = new MyDb;

$name = 'Guerre Mondiali';
$cat = $db->getCategoryByName($name);
$parent = $db->getParent($cat);

?>
<p>
La categoria principale di <?php echo $cat->name ?> (ID: <?php echo $cat->category_id ?>) è <?php echo $parent->name ?> (ID: <?php echo $parent->category_id ?>)
</p>

db.sql

create table categories (
    category_id int not null AUTO_INCREMENT,
    name varchar(100),
    parent_id int not null,
    primary key (category_id),
);

insert into categories(category_id, name, parent_id) values (0, '/', null);
insert into categories(category_id, name, parent_id) values (1, 'Storia', 0);
insert into categories(category_id, name, parent_id) values (2, '1900', 1);
insert into categories(category_id, name, parent_id) values (3, 'Guerre Mondiali', 2);
insert into categories(category_id, name, parent_id) values (4, 'Turismo', 0);
insert into categories(category_id, name, parent_id) values (5, 'Itinerari', 4);