Fifty Shades of Gray

<?php
$currentColor = 0;
for ($rows = 0; $rows < 5; $rows++) {
    for ($cols = 0; $cols < 10; $cols++) {
        echo "<div style=\"background-color: rgb($currentColor, $currentColor, $currentColor);\"></div>";
        $currentColor += 5;
    }
    echo "<br>";
    $currentColor++;
}
?>