<?php
$px = isset($_GET['px']) ? $_GET['px'] : 0;
$px = preg_replace('/[^0-9]/isU', '', $px);

$py = isset($_GET['py']) ? $_GET['py'] : 0;
$py = preg_replace('/[^0-9]/isU', '', $py);

if ($px<1) {
    $px = 5;
}
if ($py<1) {
    $py = 5;
}

if ($px>20) {
    $px = 20;
}
if ($py>20) {
    $py = 20;
}

    $width  = 100;
    $height = 100;
    $im = imagecreatetruecolor($width, $height);

for ($y=0; $y<$height; $y+= $py) {
    for ($x=0; $x<$width; $x+= $px) {
        $c = imagecolorallocate($im, 200-$x, 100+$y, 100+$x-$y);
        imagefilledrectangle($im, $x, $y, $x+$px, $y+$py, $c);
    }
}

    header("Content-type: image/png");
    imagepng($im);
    imagedestroy($im);