<?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);