I'm not sure what environment you are running the project, so my help can be limited. I also do not know what your php code is.
If this is a phone app, which I am assuming it to be. I believe SnapshotURL gives the location of the snapshot on the phone's file system, e.g. /internalstorage/yourapp/snapshots or whatever. Then if your php script gets this data what is it to do? I doesn't actually have the file you're telling it, only its path on the phone it was taken on. I do not know if Construct has a proper upload feature, but what you could possibly do it retrieve the data from the snapshot, the actual 1's and 0's in a reconstructible format, an image raster. Then you can get this data and post it to your php page for the php to then reconstruct the data from the raster and display the image.
I'm no expert in this area so this could all be completely wrong, but I hope it helps.