PHP舍入数到小数点后6位

Question

I need to return a result of (1 / n!) * (1! + 2! + 3! + ... + n!), n>=1. This is a CodeWars challenge! The code below returns 1.146652 for n = 8, but the correct result is 1.1466510000000001 or 1.146651.

How can I truncate this number correctly?

function factorial($val){
  $factor = 1;
  for($i=1;$i<=$val;$i++){
    $factor *= $i;
  }
  return $factor;
}

function going($n) {
  $val = 1/factorial($n);
  $somatorio = 0;
  for($i=1;$i<=$n;$i++){
    $somatorio += factorial($i);
  }

  return round($val * $somatorio,6);
}