euler_problem_2.php

<?php
/*
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.

O,E,O,O,E,O,O,E,O,O,

A number in the Fibonacci sequence can only be even if its 2 preceding terms are either both even or both odd, as it turns out, every 3rd term is even.

As it also turns out, there isn't really a whole lot I can do with this information.
*/

function euler_2($n) {
    $result = 0;
    $i = 1;
    while (fibonacci($i) < $n) {
        $i++;
        if (!(fibonacci($i) & 1)) {
            $result += fibonacci($i);
        }
    }
    return $result;
}

echo euler_2(4000000); // Returns 4613732

// Function to work out the fibonacci numbers. Done as an array because it is much much faster than using a recursive function.

function fibonacci($n) {
    if (!is_int($n) || $n < 1) {
        return undefined; // Didn't really need this as I'm controlling the input, but you just never know.
    }
    if ($n == 1 || $n == 2) {
        return $n;
    }
    $result = [1,2];
    for ($i = 2;$i < $n;$i++) {
        $result[$i] = $result[$i-1] + $result[$i-2];
    }
    return $result[$n-1];
}
?>