ShortTermMemory.php

<?php

namespace App\Some\Path;

class ShortTermMemory
{
    private $store = [];

    public function set($key, $value)
    {
        $this->store[$key] = $value;
    }

    public function get($key)
    {
        return $this->store[$key] ?? null;
    }

    public function has($key)
    {
        return isset($this->store[$key]);
    }

    public function remember($key, $callable)
    {
        if ($this->has($key)) {
            return $this->get($key);
        }

        $result = $callable();

        $this->set($key, $result);

        return $result;
    }
}

// and a helper function...
function stm($key = null, $default = null)
{
    $stm = resolve(\App\Some\Path\ShortTermMemory::class); // I've bound this to a singleton in an SP

    if (is_null($key)) {
        return $stm;
    }

    if (is_null($default)) {
        return $stm->get($key);
    }

    return $stm->has($key) ? $stm->get($key) : $default;
}