If compiled in standalone cgi mode, when the script exits, php only needs to free resources, not other arrays or strings or items (unless we ever support object destructors).

It can take a LONG time to free a hundred thousand or so elements in nested arrays, 4 or 5 times longer than it takes to create then.

See these threads in phpdev mailing list:
Re: [PHP-DEV] maybe serious error in RC3 memory manager
RE: [PHP-DEV] CGI quick cleanup

Sample code:
Note how long it takes to exit after finishing.

#! /usr/bin/php -q
<?php

ini_Set("max_execution_time","0");
ini_Set("memory_limit","500M");

class thingy {
   function thingy($c) {
     if ($c>0) $this->ref=&new thingy($c-1);
   }
}

$stash=array();
$max=500000;

$start=time();

for($i=0;$i<$max;$i++) {
   $r=rand(0,300);
   $stash[$r][]=&new thingy(rand(0,10));
   echo "\rUse: ".floor($i/$max*100)."% ";
}
echo "\n";

$mid=time();

$max=count($stash);
$c=0;
foreach(array_keys($stash) as $key) {
   unset($stash[$key]);
   $c++;
   echo "\rFree: ".floor($c/$max*100)."% ";
}
unset($stash);
echo "\n";

$done=time();

print "Use: ".($mid-$start)."\n";
print "Free: ".($done-$mid)."\n";


?>