Detect which image is sharper

I'm looking for a way to detect which of two (similar) images is sharper.

I'm thinking this could be using some measure of overall sharpness and generating a score (hypothetical example: image1 has sharpness score of 9, image2 has sharpness score of 7; so image1 is sharper)

I've done some searches for sharpness detection/scoring algorithms, but have only come across ones that will enhance image sharpness.

Has anyone done something like this, or have any useful resources/leads?

I would be using this functionality in the context of a webapp, so PHP or C/C++ is preferred.