Write a simple PSR-4 autoload

$mkdir App
$mkdir App/Data
$code App/Data/Database.php

<?php
namespace App\Data;

class Database {
    public function __construct()
    {
        echo "the init\n";
    }

    public function sayHi()
    {
        echo "create database\n";
    }
}

{
    "autoload": {
        "psr-4": {
            "App\\":"App/"
        }
    }
}

<?php
require __DIR__ . '/vendor/autoload.php';
use App\Data\Database;

$obj  = new Database();

echo $obj->sayHi();
?>

the init
create database