I saw this mentioned somewhere else but if you do

$var = NULL

then I've noticed less memory usuage than with unset(). In fact, 
unset didn't do anything.

This might be useful if you're doing a php-gtk app, thats starting 
to consume significant memory over a long period of time. This was 
the code I used to test

// Check memory before here

for($i = 0; $i < 100; $i++)
{
  $dialog = &new GtkDialog();
  $dialog->realize();
  $dialog->destroy();

  $dialog = NULL;
  //unset($dialog);
}

// Check memory after here

Doing a difference between after and before results in:

Using destroy() and unset() ->  ~31kb
Using $dialog = NULL -> ~13 kb

The expected memory usuage should be 0kb or around there.