InvalidPlanClassException.php

<?php

declare(strict_types=1);

namespace App\Plans;

use \Exception;

class InvalidPlanClassException extends Exception
{
    public static function for(string $classname): self
    {
        return new self('class with the name ' . $classname . ' did not extend/implement Plan');
    }
}

MissingPlanClassException.php

<?php

declare(strict_types=1);

namespace App\Plans;

use \Exception;

class MissingPlanClassException extends Exception
{
    public static function for(string $classname): self
    {
        return new self('A class with the name ' . $classname . ' could not be found');
    }
}

PlanFactory.php

<?php

declare(strict_types=1);

namespace App\Plans;

class PlanFactory
{
    public function createPlan(string $plan = 'free'): Plan
    {
        $planName = 'App\\Plans\\' . ucwords($plan) . '\\Plan';
        
        if (!class_exists($planName)) {
            throw MissingPlanClassException::for($planName);
        }

        if (!$planName instanceof Plan) {
            throw InvalidPlanClassException::for($planName);
        }

        return new $planName();
    }
}