How to create an image on the fly?

Hi.

I've created an image with this code

Header("Content-Type: image/png");
$image = ImageCreate($W,$H);
$white = ImageColorAllocate($image,255,255,255);
$black = ImageColorAllocate($image,0,0,0);
ImageLine($image,0,0,$W,$H,$black);
ImagePng($image);
ImageDestroy($image);

but now I wish to add a line

echo "Hello";

before the image code, then I only get garbage, the header command is
probably invalid now. So how do I create the image without having to save
the image to a file?
.