<pre>
<?php

// copy on write in php

// references:
// http://php.net/manual/en/features.gc.php
// http://www.tuxradar.com/practicalphp/18/1/11

function echoMemUsage() {
    echo round((float)memory_get_usage()/1048576, 2)." MB\n";
}

echoMemUsage(); // 0.08 MB

// declare a long string
$str_a = str_repeat("helloworld", 100000);

echoMemUsage(); // 1.03 MB

// str_b is only given a reference to str_a
$str_b = $str_a;

echoMemUsage(); // 1.03 MB

// str_b is `modified' and COW comes in
$str_b = $str_b."";

echoMemUsage(); // 1.99 MB

// remove str_b from memory
// note that mem usage decrease
unset($str_b);

echoMemUsage(); // 1.03 MB

// -----------------------------------------
// before php 5.3.0

$z = array();
$z[0] = $str_a;
$z[1] = &$z; // reference to itself
unset($str_a);

echoMemUsage(); // 1.03 MB

unset($z);

echoMemUsage(); // 1.03 MB

// at this stage, we have unset all variables
// but there is a memory leak!

?>
</pre>