<?php

$height = isset($_GET['height']) ? $_GET['height'] : 80;
$width = isset($_GET['width']) ? $_GET['width'] : 80;
// top | left
$dir = isset($_GET['direction']) ? $_GET['direction'] : 'top';
$start = $_GET['start'];
$end = $_GET['end'];

list($r, $g, $b) = hex2rgb($start);
list($r2, $g2, $b2) = hex2rgb($end);

$img = imagecreatetruecolor($width, $height);
$pixel = $dir == 'top' ? $height : $width;
for($i = 0; $i < $pixel; $i++) {
    $color = imagecolorallocate($img,
        round($r - ($r - $r2)/($pixel-1)*$i),
        round($g - ($g - $g2)/($pixel-1)*$i),
        round($b - ($b - $b2)/($pixel-1)*$i));
    imageline($img,
        $dir == 'top' ? 0 : $i, $dir == 'top' ? $i : 0,
        $dir == 'top' ? $width : $i, $dir == 'top' ? $i : $height,
        $color);
}

// output image
header('Content-Type: image/png');
imagepng($img);
imagedestroy($img);


function hex2rgb($color) {
     return array_map(function($val) {
         return hexdec($val);
     }, str_split($color, 2));
}