Quicky to plain text calendar generation

cal.php

<?php

$tz = new DateTimeZone('UTC');
$now = (new DateTimeImmutable('now', $tz))->setTime(12, 0, 0, 0);
$start = filter_input(INPUT_GET, 'start', FILTER_VALIDATE_REGEXP, ['options' => ['regexp' => '@\d{4,}-\d\d-\d\d@']]);
$end = filter_input(INPUT_GET, 'end', FILTER_VALIDATE_REGEXP, ['options' => ['regexp' => '@\d{4,}-\d\d-\d\d@']]);

if (
    is_string($start) &&
    ($startParts = array_map('intval', explode('-', $start))) &&
    checkdate($startParts[1], $startParts[2], $startParts[0])
) {
    $start = $now->setDate($startParts[0], $startParts[1], $startParts[2]);
} else {
    $start = $now->setDate(intval($now->format('Y')), 1, 1);
}
if (
    is_string($end) &&
    ($endParts = array_map('intval', explode('-', $end))) &&
    checkdate($endParts[1], $endParts[2], $endParts[0])
) {
    $end = $now->setDate($endParts[0], $endParts[1], $endParts[2]);
} else {
    $end = $now->setDate(intval($now->format('Y')), 12, 31);
}
if ($start->diff($end)->invert === 1) {
    [$start, $end] = [$end, $start];
}
// Set start to the Monday before:
$start = $start->modify(sprintf('-%d days', intval($start->format('N')) - 1));
// Set end to the Sunday after:
$end = $end->modify(sprintf('+%d days', 7 - intval($end->format('N'))));

$current = $start;
$eol = "\n";
$month = null;
$year = null;

header('Content-Type: text/plain; charset=UTF-8');

echo '         M  T  W  H  F  S  X' . $eol;
echo '----------------------------' . $eol;
if (isset($_GET['notes'])) {
    $eol = "  ____________________________\n";
}
while ($current->diff($end)->invert === 0) {
    if ($current->format('N') === '1') {
        echo $current->format('W');
        echo ' ';
        if ($month !== $current->modify('+6 days')->format('M')) {
            $month = $current->modify('+6 days')->format('M');
            echo strtoupper($month) . ' ';
        } else if ($year !== $current->modify('+6 days')->format('Y')) {
            $year = $current->modify('+6 days')->format('Y');
            echo $year;
        } else {
            echo '    ';
        }
    }
    echo ' ' . $current->format('d');
    if ($current->format('N') === '7') {
        echo $eol;
    }
    $current = $current->modify('+1 day');
}