first of all I don’t have alot of experience with WordPress and it’s plugins so my question/problem may be stupid.
I want to make a plugin that cannot be modified. As far as I know nothing can stop the user to go and modify the .php file of my plugin on his server.
So the problem is: how can I check if the source code of my plugin.php file has been modified by the user ?
I’ve no doubt a lot of people will want to know why you don’t want people to modify your plugin, but I’m sure you have your reasons.
Short answer, there isn’t a way to stop people editing the files you package in your plugin.
However, a lot of plugins simply offer users an embed code which bring in functionality from external source. ShareThis do something along these lines, where the user effectively just selects options for the embed code. Maybe this approach would suit your needs?
There are guidelines relating to this kind of plugin behavior here: http://wordpress.org/extend/plugins/about/guidelines/