$a = microtime(true);
$d = array(2, 3, 5, 7);
$i = 2;
for ($b = 11; $b < 1000; $b += $i, $i = 6 - $i) {
    if ($b % 5 && $b % 7) {
        if ($b > 49) {
            $f = sqrt($b);
            for ($c = 4; $x = $d[$c], $x <= $f; $c++)
              if (!($b % $x)) continue 2;
        }
        $d[] = $b;
    }
}
printf("%01.5f\n",microtime(true)-$a);
print_r($d);