<?php
/**
 *
 */

$input = file('input', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
$input = array_map('intval', $input);

$increased       = 0;
$previousReading = array_sum(array_slice($input, 0, 3));
for ($i = 1; $i < count($input) - 2; $i++) {
    $currentReading = array_sum(array_slice($input, $i, 3));
    if ($currentReading > $previousReading) {
        $increased++;
    }
    $previousReading = $currentReading;
}
echo $increased; // 1597