Get the reference count of an object in PHP?

Sean's debug_zval_dump function looks like it will do the job of telling you the refcount, but really, the refcount doesn't help you in the long run.

You should consider using a bounded array to act as a cache; something like this:

<?php
class object_cache {
   var $objs = array();
   var $max_objs = 1024; // adjust to fit your use case

   function add($obj) {
      $key = $obj->getKey();
      // remove it from its old position
      unset($this->objs[$key]);
      // If the cache is full, retire the eldest from the front
      if (count($this->objs) > $this->max_objs) {
         $dead = array_shift($this->objs);
         // commit any pending changes to db/disk
         $dead->flushToStorage();
      }
      // (re-)add this item to the end
      $this->objs[$key] = $obj;
   }

   function get($key) {
      if (isset($this->objs[$key])) {
          $obj = $this->objs[$key];
          // promote to most-recently-used
          unset($this->objs[$key]);
          $this->objs[$key] = $obj;
          return $obj;
      }
      // Not cached; go and get it
      $obj = $this->loadFromStorage($key);
      if ($obj) {
          $this->objs[$key] = $obj;
      }
      return $obj;
   }
}

Here, getKey() returns some unique id for the object that you want to store. This relies on the fact that PHP remembers the order of insertion into its hash tables; each time you add a new element, it is logically appended to the array.

The get() function makes sure that the objects you access are kept at the end of the array, so the front of the array is going to be least recently used element, and this is the one that we want to dispose of when we decide that space is low; array_shift() does this for us.

This approach is also known as a most-recently-used, or MRU cache, because it caches the most recently used items. The idea is that you are more likely to access the items that you have accessed most recently, so you keep them around.

What you get here is the ability to control the maximum number of objects that you keep around, and you don't have to poke around at the php implementation details that are deliberately difficult to access.

It seems like the best answer was still getting the reference count, although debug_zval_dump and ob_start was too ugly a hack to include in my application.

Instead I coded up a simple PHP module with a refcount() function, available at: http://github.com/qix/php_refcount