$ php test1.php
Start
112
X
488
Cleanup
112
Y
480
Cleanup
112

$ cat test1.php
<?php
class X {
    private $_b;
    private $_e;
}

class Y {
    private $_behaviors;
    private $_events;
}

$base = memory_get_usage();

echo "Start\n";
echo memory_get_usage() - $base."\n";

$a = new X();
$b = new X();

echo "X\n";
echo memory_get_usage() - $base."\n";

unset($a);
unset($b);

echo "Cleanup\n";
echo memory_get_usage() - $base."\n";

$a = new Y();
$b = new Y();

echo "Y\n";
echo memory_get_usage() - $base."\n";

unset($a);
unset($b);

echo "Cleanup\n";
echo memory_get_usage() - $base."\n";