composer.json.example.forpsr4

{
    "name": "Myapp/Myapp",
    "description": "",
    "keywords": ["secret"],
    "require": {
        "php": ">=5.5.0",
        "slim/slim": "3.0",
        "slim/php-view": "^2.0",
        "monolog/monolog": "^1.17",
        "wepay/php-sdk": "^0.2",
        "paypal/adaptivepayments-sdk-php": "v3.9.2",
        "phpunit/phpunit": "5.4.2",
        "davidepastore/slim-validation": "^0.5.0",
        "stripe/stripe-php": "^6.6"
    },

  "autoload": {
    "psr-4": {
      "Myapp\\": "src/Myapp",
      "PayPal\\": "vendor/paypal/adaptivepayments-sdk-php/lib/PayPal"
    }
  }
}

notice the folders and files are (from the top of my head)

/yourdevdir/
     composer.json
     src/Myapp/MyTestClass.php
     src/Myapp/MyTestClassB.php

MyTestClass.php

namespace Myapp;
class MyTestClass{

}

MyTestClassB.php

namespace Myapp;
use Myapp\MyTestClass;

class MyTestClassB{
   public function test(){
      $x = new MyTestClass();
   }
}