PDO lastInsertId（）不会打印任何值

Question

I've done create method which should add new User object into database and on success it should assign id from the auto_increment field in the database.

Everything works except the fact that PDO lastInsertId() does not output any value.

Here is my code:

public function create() {
    $stmt = $this->connect()->prepare(
                'INSERT INTO users 
                (username, password, first_name, last_name) 
                VALUES (?, ?, ?, ?)');
    $stmt->execute(
            [$this->username, $this->password, $this->first_name, $this->last_name]);

    if ($stmt) {
        echo $this->connect()->lastInsertId(); 
    }
}

Method connects() refers to method in Database class which returns PDO class object.