The question is - does Player upgrade buildings, or does a service do this, or does the building upgrade itself and deduct costs from Player?

File1.php

<?php
class Player
{
  public function upgrade(Building $building)
  {
    if (!$this->canAfford($building)) {
      throw new DamnImBroke;
    }
    
    $this->deduct($building->upgradeCosts());
    $building->upgrade();
  }
}

File2.php

<?php
class BuildingUpgrader
{
  public function improve(Player $player, Building $building)
  {
    if (!$player->canAfford($building->upgradeCosts())) {
      throw new DamnImBroke;
    }
    
    $player->deduct($building->upgradeCosts());
    $building->upgrade();
  }
}

File3.php

<?php
class Building
{
  publi function upgrade(Player $player)
  {
    if (!$player->canAfford($building->upgradeCosts())) {
      throw new DamnImBroke;
    }
    
    $player->deduct($this->upgradeCosts());
    $this->upgrade();
  }
}

Hey kirk, I worked on a similar project and i'm interested of what you ended up doing.

Cheers,
Hugo.