example.php

<?php

namespace Project\Planets;

class Planet
{
    protected $name;

    /**
     * @var Resource[]
     */
    protected $resources = [];

    public function __construct($name)
    {
        $this->name = $name;
    }

    public function addResource(Resource $resource)
    {
        $this->resources[$resource->getName()] = $resource;
    }

    public function hasEnoughResource(Resource $resource)
    {
        $resourceName = $resource->getName();

        if(!array_key_exists($resource->getName(), $this->resources)) {
            return false;
        }

        return ($resource->getValue() <= $this->resources[$resourceName]->getValue());
    }
}

class Resource
{
    protected $name;

    protected $value;

    public function __construct($name, $value)
    {
        $this->name = $name;
        $this->value = $value;
    }

    public function getName()
    {
        return $this->name;
    }

    public function getValue()
    {
        return $this->value;
    }
}

class Oxygen extends Resource
{
    public function __construct($value)
    {
        parent::__construct('oxygen', $value);
    }
}


$planet = new Planet('Sol');
$planet->addResource(new Resource('Metal', 100));
$planet->addResource(new Resource('Oxygen', 150));

$planet->hasEnoughResource(new Resource('Oxygen', 170));

// better is this:
/**
 * $planet->addResource(new Oxygen(20));
 */