Is there a way to ensure a user is still linking to my website without manually check

[Smudly]

I am working on a link exchange section on my site. The user can gain special access on my site for as long as they link to my website from theirs. Is there a way to create a php or java script that automatically checks if they are still linking to my website? Any ideas on how I could do this?

[dagon]

file_get _contents() and a regular expression to find your url. google search api with inurl may work.