Problem

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

Solution

$sum = 2;
$prev = 1;
$curr = 2;

for ($temp = 0; $temp < 4000000; $temp = $prev + $curr) {
  $prev = $curr;
  $curr = $temp;
  if ($temp % 2 == 0) {
    $sum += $temp;
  }
}

echo $sum;