使用PHP生成此图像

I want to generate this image in PHP with different colors for a skinning module in my app. How do I go about it?

image(2x40 pixels):

magnified 4x

The original image was generated using the gradient tool in paint.net with base color 00137F.

I've used this gradient class before, you may find it helpful: http://planetozh.com/blog/my-projects/images-php-gd-gradient-fill/