PHP Composer问题

It's been quite a while since I have done any heavy PHP development. I am trying to bring an older library up to date with composer and namespaces and am having a hard time fully understanding things.

Here is my composer.json

{
    "description": "MyClass",
    "require": {
        "php": ">=5.3.0",
        "guzzlehttp/guzzle": "~6.0"
    },
    "autoload": {
        "psr-4": {
            "MyClass\\": "src/"
        }
    }
}

I am trying to write some examples on how to use the updated library, but am having trouble getting my code to run. Here is my example file:

<?php

use MyClass;

$account_id = 123; // Replace with your account id
$public_key = '09876'; // Replace with your public key
$private_key = 'privkey'; // Replace with your private key
$klass = new MyClass($account_id, $public_key, $private_key);

echo $klass->myMethod();

Every time I run the above code I get the following error:

Warning: The use statement with non-compound name 'MyClass' has no effect
PHP Fatal error:  Class 'MyClass' not found