php FOR 循环优化有关问题

[User:root Time:21:21:46 Path:/home/liangdong/php]$ php permutation.php 
Total time used : 9738642ms
[User:root Time:21:21:56 Path:/home/liangdong/php]$ cat permutation.php 
<?php $beg_time = microtime(true);
$beg_time *= 1000 * 1000;

$table = range(1, 33);
$temp = array();
$fp = null;

function _get_permutations($index, $num) {
        global $temp;
        global $total;
        global $table;
        global $fp;

        if ($num == $total) {
                global $perms;
                fwrite($fp, json_encode($temp) . PHP_EOL);
                return;
        }

        for ($ndx = $index; 33 - $ndx + $num >= $total; ++ $ndx) {
                $temp[$num] = $table[$ndx];
                _get_permutations($ndx + 1, $num + 1);
        }
}

function get_permutations($n = 6) {
        global $total;
        global $perms;
        global $fp;

        if ($n  33) {
                return false;
        }

        $total = $n;
        $fp = fopen("data.txt", "w");
        _get_permutations(0, 0);
        fclose($fp);
        return true;
}

get_permutations();

$end_time = microtime(true);
$end_time *= 1000 * 1000;
echo "Total time used : " . ($end_time - $beg_time) . "ms" . PHP_EOL;

?&gt; <div class="clear"></div>