dynamic png not working

[Chris Cortese]

I just noticed on one of my sites that my php script that calls 
header("Content-type: image/png") and imagepng(), etc., is not creating 
a png image.  This was working before but I've not only switched to 
nginx from Apache, I've also switched to a new server, new build of PHP, 
etc...

Do I need to do something special with the nginx config to make this happen?