Usar array estoura a memória do PHP.

run.php

<?php

function printResults()
{
    $startOfRequest = $_SERVER['REQUEST_TIME'];
    $memoryUsed = formatBytes(memory_get_usage());
    $memoryPeak = formatBytes(memory_get_peak_usage());
    echo <<<EOT
Memory usage: $memoryUsed
Memory peak: $memoryPeak

EOT;
}

function createRandonString()
{
    return md5(mt_rand());
}

function populateIteator($iterator, $length=999999)
{
    $result = $iterator;
    for($i=0; $i<$length; $i++)
        $result[$i] = createRandonString();

    return $result;
}

function doSomethingWithIterator($iterator)
{
    // whatever
}

// http://stackoverflow.com/questions/2510434/format-bytes-to-kilobytes-megabytes-gigabytes
function formatBytes($bytes, $precision = 2)
{
    $kilobyte = 1024;
    $megabyte = $kilobyte * 1024;
    $gigabyte = $megabyte * 1024;
    $terabyte = $gigabyte * 1024;

    if (($bytes >= 0) && ($bytes < $kilobyte)) {
        return $bytes . ' B';

    } elseif (($bytes >= $kilobyte) && ($bytes < $megabyte)) {
        return round($bytes / $kilobyte, $precision) . ' KB';

    } elseif (($bytes >= $megabyte) && ($bytes < $gigabyte)) {
        return round($bytes / $megabyte, $precision) . ' MB';

    } elseif (($bytes >= $gigabyte) && ($bytes < $terabyte)) {
        return round($bytes / $gigabyte, $precision) . ' GB';

    } elseif ($bytes >= $terabyte) {
        return round($bytes / $terabyte, $precision) . ' TB';
    } else {
        return $bytes . ' B';
    }
}

$currentScript = array_shift($argv);
$runProgram = array_shift($argv);
$arrayLength = array_shift($argv) ?: 999999;
switch($runProgram) {
    case 'array':
        $array = populateIteator(array(), $arrayLength);
        doSomethingWithIterator($array);
        doSomethingWithIterator($array);
        printResults();
        break;
    case 'object':
        $array = populateIteator(new ArrayObject($arrayLength), $arrayLength);
        doSomethingWithIterator($array);
        doSomethingWithIterator($array);
        printResults();
        break;
    default:
        echo "Usage: $currentScript <array|object> [length=999999]";
        echo PHP_EOL;
        break;
}