What I did.

Step 1

• First, I saw that the code couldn't work because of simple errors like

class ElectronicItems 
{
     /**
     * @param string $type
     * @return array
     */
    public function getItemsByType($type)
    {
        /**
        * ElectronicItem::$types has private visibility
        */
        if (in_array($type, ElectronicItem::$types)) {
            $callback = function ($item) use ($type) {
                return $item->type == $type;
            };
            $items = array_filter($this->items, $callback);
        }

        return false;
    }
}

class ElectronicItem
{
    private static $types = array(
        self::ELECTRONIC_ITEM_CONSOLE,
        self::ELECTRONIC_ITEM_TELEVISION,
        self::ELECTRONIC_ITEM_MICROWAVE
    );
}

• There were several methods with a broken contract. The return type was declared as an array but really was returned a boolean.
• Added docker for local environment

Step 2

Adjusted the code to execute with PHP 8

Step 3

Big refactoring.

• Changed ElectronicItem class into an interface.
• Implemented all require devices from task
• Changed methods in the class ElectronicItems

Step 4

• Added symfony's component symfony/console as a way of representation of working business logic
• Added package myclabs/php-enum to turn electronic types into Enum object
• Added factory to easily create particular electronic items
• Created a console command to execute a required scenario to buy several items and calculate their price

Step 5

Supplemented the existing scenario with the new one from question 2

Step 6

Wrote unit tests for business logic

To run the script:

docker-compose run app bin/console purchase