is there a way to add some extra metadata to a PNG file and then retrieve it with PHP ?
like Comment field.
If you have ImageMagick installed, it comes with two utilities: mogrify and identify.
To add comments:
$ mogrify.exe -comment "My test comment" plogo.png
To retrieve comments:
$ identify.exe -verbose plogo.png | grep -i "comment:"
comment: My test comment
I don't know if mogrify/identify functions are available as PHP libraries, but you can always use the php system command.
来源：https://stackoverflow.com/questions/4497206/how-to-add-extra-meta-data-to-png