Collatz conjecture test

collatz.php

<?php
/**
 * Collatz Conjecture Test
 *
 * This will generate a file showing each step taken to reach 1 following
 * the Collatz Conjecture with integers between 2 and 2^20. Note that the
 * resulting file will be over 2 GB, so changing pow(2, 20) to a smaller
 * number like pow(2, 16) may be preferable.
 *
 * @author Alan Hardman <alan@phpizza.com>
 * @see    http://en.wikipedia.org/wiki/Collatz_conjecture
 */
$f = fopen("collatz.txt", "w");
for($i = 2; $i < pow(2, 20); $i++) {
    $n = $i;
    echo $i, "\n";
    while($n != 1) {
        fwrite($f, "$i: $n\n");
        if($n % 2) {
            $n = $n * 3 + 1;
        } else {
            $n = $n / 2;
        }
    }
    fwrite($f, "$i: 1 SUCCESS\n");
}
fclose($f);