Project Euler - problem 2

<?php
    // My answer to http://projecteuler.net/problem=2
    $tot = 0;
    $nums = array(1);
    $a = 0;
    while ($a < 4000000) {

        $a = array_sum($nums);

        if ($a%2==0) {
            $tot += $a;
        }

        array_push($nums,$a);

        if (count($nums)>2) {
            array_shift($nums);
        }


    }
    echo $tot;
    exit;
?>