Make a wrapper around the PHP script, which:
- performs the stuff (If I understand well, it's an image creation),
- then redirects (301 Moved Permanently) to the result image,
- (also, the image should be purged some day).
So you can refer to this service (the PHP script) with a simple HTTP request, from anywhere, you can test it with browser, use from Python prg, you need just download the image the usual way.
Also, if you have a such standalone sub-system, don't feel bad about implement it with different language/technique. It has several advantages, e.g. you can install that service on a different host.
Recommended reading: Service-Oriented Architecture on Wikipedia.