<?php

someFactory(InstanceManager $im, $name, $isShared = false)
{
    // calculate the service
    // ...
    // and do this:
    if ($isShared) {
        $im->set($name, $result);
    }

    // or:
    if ($isShared) {
        $this->service = $result;
    }

    // and on subsequent calls, return it early:
    if ($this->service) {
        return $this->service;
    }
}