What is the json_encode data type in Interface PHP

I am creating my personal Todo App using OOP PHP and Composer Autoload.

I am using Interface to Declare the Classes.

In some of my classes has return type json_encode()

LIKE THIS:

<?php
include ("../db.php");

class ActiveTodo {

    private $connection_string ;

    public function getActiveList() {
        $sql = "SELECT * FROM todo WHERE todo_status = 0";
        $result = mysqli_query($this->connection_string, $sql);
        return $result ? $result->fetch_all(MYSQLI_ASSOC) : [];
    }
}

$todo = new ActiveTodo;
echo json_encode($todo->getActiveList());

?>

I have declared Interface like this.

<?php
namespace todo\app;

interface ILastid extends IDb{
    public function fetchLastId();
}

Now what will be the return type for json_encode()? If we have int return then we gave:

public function getId() : int;