PHP Garbage Collection for Singleton Properties - php

Question

(Specifically, I'm working inside Magento, but this is a general PHP question.)
I have a singleton class with an array which stores large objects. Please consider the following:
protected $_categoryObjectCache = array();
/**
* #param $key
* #param $value
*/
public function setCategoryObjectCache($key, $value) {
$this->_categoryObjectCache[$key] = $value;
if ($this->nearingMemoryLimit()) {
unset($this->_categoryObjectCache);
if (function_exists('gc_collect_cycles')) {
gc_collect_cycles();
}
$this->_categoryObjectCache = array();
}
}
/**
* #return bool
*/
protected function nearingMemoryLimit() {
$memory_limit = ini_get('memory_limit');
if (preg_match('/^(\d+)(.)$/', $memory_limit, $matches)) {
if ($matches[2] == 'M') {
$memory_limit = $matches[1] * 1024 * 1024; // nnnM -> nnn MB
} else if ($matches[2] == 'K') {
$memory_limit = $matches[1] * 1024; // nnnK -> nnn KB
}
}
return ((memory_get_usage() / $memory_limit) > 0.75);
}
(The method setCategoryObjectCache() doesn't get called very often. Usually I'm just reading from that array with a method called getCategoryObjectCache())
I built this singleton cache as part of an importer which goes through thousands of products and assigns them to various various categories. However, this requires loading each category model and using it's built in methods to look up parent/child relationships, status, etc. Looking up each category that a product wants to relate to, over and over for every product incurred thousands of duplicate model instantiations for the same category. This cache solves that problem.
The issue though, is that I'm worried about the cache getting too big. Magento is notorious for having performance issues because of these huge model objects. So I wrote the above code to look at the current scripts memory usage and clear the cache if the script gets too close to maxing out memory.
What I don't know, is whether this is really going to work. PHP garbage collection is a big ball of mystery to me, and I'm not sure if I could just call $this->_categoryObjectCache = array(); and expect PHP to clean up, or if I really needed to call unset($this->_categoryObjectCache); and gc_collect_cycles();
How can I make sure to clear out the RAM when my cache starts getting too big?