BoletoPayment.php

<?php
namespace App\Student;

class BoletoPayment implements Payment
{
  // todo
}

CreditCardPayment.php

<?php
namespace App\Student;

class CreditCardPayment implements Payment
{
  // todo
}

Payment.php

<?php
namespace App\Student;

interface Payment
{
}

Student.php

<?php

namespace App\Student;

use Doctrine\Common\Collections\{ArrayCollection, Collection};
use DateTime;

class Student
{
  protected $id;
  
  protected $name;
  
  /**
   * @var Collection|Subscription[]
   */
  protected $subscriptions;
  
  public function __construct(int $id, string $name)
  {
    $this->id = $id;
    $this->name = $name;
    $this->subscriptions = new ArrayCollection();
  }
  
  public function subscribe(DateTime $expireAt)
  {
    $this->subscriptions->add(new Subscrption($expireAt));
  }
  
  /**
   * @var Subscription[]
   */
  public function getSubscriptions(): array
  {
    return $this->subscriptions->toArray();
  }
}

Subscription.php

<?php

namespace App\Student;

use DateTime;
use Doctrine\Common\Collections\{ArrayCollection, Collection};

class Subscription
{
  /**
   * @var DateTime
   */
  protected $expireAt;
  
  /**
   * @var Collection|Payment[]
   */
  protected $payments;
  
  public function __construct(DateTime $expireAt)
  {
    $this->expireAt = $expireAt;
    $this->payments = new ArrayCollection();
  }
  
  public function addPayment(string $type, array $data)
  {
    switch($type) {
      case 'boleto':
        $payment = new BoletoPayment($data['ournumber']); // etc, etc
        break;
      case 'creditcard':
        $payment = new CreditCardPayment($data['cardnumber']); // etc, etc
        break;
    }
    
    $this->payments->add($payment);
  }
}