<?php

$increased   = 0;
$inputStream = fopen('input', 'r');
$horizontal  = $depth = 0;

while (($line = fgets($inputStream)) !== false) {
    [$instruction, $value] = explode(' ', trim($line));
    match ($instruction) {
        'down' => $depth += (int)$value,
        'up' => $depth -= (int)$value,
        'forward' => $horizontal += (int)$value,
    };
}
echo $depth * $horizontal; // 1635930