PHP array-backed LRU cache with capacity

<?php
final class LruCache
{
    private array $items = [];
    public function __construct(private int $capacity)
    {
        if ($capacity < 1) throw new InvalidArgumentException('Capacity must be positive');
    }
    public function get(string $key): mixed
    {
        if (!array_key_exists($key, $this->items)) return null;
        $value = $this->items[$key];
        unset($this->items[$key]);
        return $this->items[$key] = $value;
    }
    public function put(string $key, mixed $value): void
    {
        unset($this->items[$key]);
        $this->items[$key] = $value;
        while (count($this->items) > $this->capacity) unset($this->items[array_key_first($this->items)]);
    }
}