<?php
$recipies = "37";
$elf1 = 0;
$elf2 = 1;

$after = '939601';
$al = strlen($after);

$pos = 0;
$part1 = false;
$part2 = false;

while(++$pos>0)
{
    $r1 = (int)$recipies[$elf1];
    $r2 = (int)$recipies[$elf2];

    $sum = $r1 + $r2;

    $recipies .= $sum;

    $elf1 += 1 + $r1;
    $elf2 += 1 + $r2;

    $elf1 %= strlen($recipies);
    $elf2 %= strlen($recipies);

    if(!$part1 && strlen($recipies)>= ((int)$after)+10)
    {
        echo printf('Part 1: %d', substr($recipies, (int)$after, 10));
        echo PHP_EOL;
        $part1 = true;
    }

    $s = $pos - $al;
    if(!$part2 && substr($recipies, $s, $al) === $after)
    {
        echo printf('Part 2: %d', $s);
        echo PHP_EOL;
        $part2 = true;
    }

    if($part1 && $part2)
        exit();
}