Fabonacci Problem

<?php
    $preTerm = 1;
    $nextTerm = 2;
    $fibSum = 3;
    $fibEvenNumberSum = 2;
    for($i = 1 ; $i < 40 ; $i++)
    {
        $fibSum = $preTerm +$nextTerm;
        if($fibSum < 4000000)
        {
            $preTerm = $nextTerm;
            $nextTerm = $fibSum;
            if( $fibSum % 2 == 0 )
                $fibEvenNumberSum += $fibSum;
        }
        else
            break;
    }
    echo $fibEvenNumberSum;
?>

// output is: 4613732