unset-test.php

<style>
iframe { border:none;border-bottom:1px solid #ccc;width:100%;height:250px; }
body, * { font-family:Consolas,"Lucida Console","Lucida Typewritter","Courier New",Courier,monospace; }
</style>
<?php

class FOO {
    public $f;
}

class BAR {
    public $f;
}

$memory = 0;
$increases = 0;

gc_enable();

if (isset($_REQUEST['test'])) {
    for ($n=1;$n<1000000 && $increases < 40 && $memory < (24 * 1024 * 1024);$n++) {
        $a = new FOO();
        $b = new BAR();
        
        $a->f = $b;
        $b->f = $a;
        
        $current = memory_get_usage(true);
        
        if ($current != $memory) {
            $memory = $current;
            $increases++;
            print "Memory usage: " . number_format($memory) . " bytes at cycle $n<br>";
        }
        
        if (isset($_REQUEST['unset-all'])) {
            unset($a->f);
            unset($b->f);
        }
        
        if (isset($_REQUEST['unset']) || isset($_REQUEST['unset-all'])) {
            unset($a);
            unset($b);
        }
    }
    
    echo "Total cycles: <strong>$n</strong><br>";
    echo "Total garbage collection cycles: " . gc_collect_cycles();
}
else {
    ?>
<h2>No unset</h2>
<iframe src="test.php?test"></iframe>
<h2>Unset variables</h2>
<iframe src="test.php?test&amp;unset"></iframe>
<h2>Unset variables and references</h2>
<iframe src="test.php?test&amp;unset-all"></iframe>
    <?php
}