Shoutbox

In fact, that's very easy to do. You can use the Output Control Functions of PHP for this.

1. Start buffering the output by calling:
   

   code:

   ---

   ob_start();

   ---

2. Then, let imagepng() send the image to the output. Instead of getting printed, PHP will store it in the output buffer.
   

   code:

   ---

   imagepng($main_image);

   ---

3. Now, you read what imagepng() has outputted with ob_get_flush(). This function gives you the output AND stops the output buffering, so basically it's does the same as ob_get_contents() and ob_end_flush() together, which is interesting in our case.
   

   code:

   ---

   $png_output = ob_get_flush();

   ---

4. Yay, you have a variable containing the PNG output! You can now save it to a file and send the PNG output to the browser after all, by simply print()ing it. Don't forget your headers!

Link: Comment by trucex on PHP: Output Control.

Argh, beaten by WDZ... the shame...