Prime Number Generator

$a = microtime(true);$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);