echo "Initial: ".memory_get_usage()." bytes \n";
/* prints
Initial: 361400 bytes
*/

// let's use up some memory
for ($i = 0; $i < 100000; $i++) {
$array []= md5($i);
}

// let's remove half of the array
for ($i = 0; $i < 100000; $i++) {
unset($array[$i]);
}

echo "Final: ".memory_get_usage()." bytes \n";
/* prints
Final: 885912 bytes
*/

echo "Peak: ".memory_get_peak_usage()." bytes \n";
/* prints
Peak: 13687072 bytes
*/