php mysqli::$insert_id -> not working properly - php

Question

I wanted to insert a row and at the same time get the id generated by that query.
I'm just testing it first and so far here is what I got.
I created a class which has a database configuration like this
class Database{
private $host;
...
...
protected function connect(){
$this->host = 'localhost';
...
...
$conn = new mysqli($this->host, ...,);
if ($conn->connect_errno){
echo 'error';
};
return $conn;
}
}
A User class for the actual query and where I extend my Database class.
class User extends Database{
function createUser(...,){
$sql = "INSERT INTO t (field) VALUES ('value');";
$this->connect()->query($sql);
return $this->connect()->insert_id;
}
}
I already have data inserted in my table but the function returns 0.
Note:
I have my column ID on user table of AUTO_INCREMENT attribute.
The query is successful but not returning the correct ID.
I tried doing this without the Database class and it works fine, returning the ID generated by the query.
$mysqli = new mysqli('host', ...,);
if(result = $mysqli->query("INSERT INTO t (field) VALUES ('value');")){
return $mysqli->insert_id;
}

Answer 1

Problem is inside your Database class.
you need a $conn field.
calling connect() causes to create new connection every time.
class Database{
private $host;
private $conn;
...
...
protected function connect(){
$this->host = 'localhost';
...
...
$this->conn = new mysqli($this->host, ...,);
if ($this->conn->connect_errno){
echo 'error';
};
}
}
class User extends Database{
function createUser(...,){
$sql = "INSERT INTO t (field) VALUES ('value');";
$this->conn->query($sql);
return $this->conn->insert_id;
}
}

Answer 2

Don't do it this way! Your Datebase class is useless and it doesn't follow the best practices of OOP.
Your problem is that each time you call $this->connect() you create a new connection. The connections do not share state between themselves so you can't access the last inserted ID using a different connection. It has to be the same connection!
Don't write such classes like Database class unless you are actually going to write abstraction library, and never inherit from a Database class.
mysqli is a class in itself. To open a database connection using mysqli you only need 3 lines of code.
mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli('localhost', 'user', 'pass', 'db');
$mysqli->set_charset('utf8mb4'); // always set the charset
You can then pass the instance to any class that needs it.
class User {
private mysqli $db;
public function __construct(mysqli $mysqli) {
$this->db = $mysqli;
}
function createUser(/* ... */) {
$stmt = $this->db->prepare("INSERT INTO t (field) VALUES (?)");
$stmt->bind_param('s', $value);
$stmt->execute();
return $this->db->insert_id;
}
}
$user = new User($mysqli);
You must always use prepared statements because it allows you to parameterize your queries. Never concatenate data directly into SQL.
Lastly, do not use mysqli on its own. It is not suitable for beginners nor to be used on its own. If you are only starting to learn PHP then you should learn PDO instead of mysqli. PDO is much easier and more suitable for beginners. Start here https://phpdelusions.net/pdo