<?php
$shade = 0;

for ($row = 0; $row < 5; $row++, $shade++)
{
    for ($col = 0; $col < 10; $col++)
    {
        echo "<div style=\"background-color: rgb($shade, $shade, $shade);\"></div>";
        $shade += 5;
    }

    echo "<br>";
}
?>