Programmer's day on Friday 13 average years - JSON Voorhees

json13.php

<?php
    //Programmer's day on Friday 13 average years - JSON Voorhees
    $r=$i=0;
    for ($y=$ly=0; $y<3000; $y++) {
        $d = new DateTime();
        $d->setDate($y, 1, 256);
        if ($d->format("D") == "Fri" && $d->format('d') == 13) {
            $r += $y - $ly;
            $ly = $y;
            $i++;
            //echo $y . "<br/>";
        }
    }
    echo $r/$i;