Prime Number Generator

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