Simple limited memory cache

InMemoryCache.php

<?php

class InMemoryCache
{
    public $size = 10;

    protected $cache = [];

    /**
     * @param string $key
     * @param callable $obtainCallback
     * @return mixed|null
     */
    public function get($key, $obtainCallback)
    {
        if (!array_key_exists($key, $this->cache)) {
            $this->cache[$key] = call_user_func($obtainCallback);

            if (count($this->cache) > $this->size) {
                reset($this->cache);
                unset($this->cache[key($this->cache)]);
            }
        }

        return $this->cache[$key];
    }
}