checkerboard.php

<?php
// This solution does not care for $n <= 1
function displayCheckerBoard($n): string {
    $gridStartWithX = 'XY';
    $gridStartWithY = 'YX';

    if ($n === 2) {
        return $gridStartWithX . PHP_EOL . $gridStartWithY . PHP_EOL;
    }

    $intDiv = intdiv($n, 2);
    $remainder = $n % 2;

    $rowX = str_repeat($gridStartWithX, $intDiv) . ($remainder > 0 ? 'X' : '');
    $rowY = str_repeat($gridStartWithY, $intDiv) . ($remainder > 0 ? 'Y' : '');

    // Construct the grid by repeating the rows
    $output = str_repeat($rowX . PHP_EOL . $rowY . PHP_EOL, $intDiv);

    if ($remainder > 0) {
        $output .= $rowX . PHP_EOL;
    }

    return $output;
}

foreach (range(2, 10) as $number) {
    echo "===============" . PHP_EOL;
    echo $number . "X" . $number .  " Checkerboard:" . PHP_EOL;
    echo "===============" . PHP_EOL;
    echo(displayCheckerBoard($number));
    echo PHP_EOL;
}