Backlinks are an important part of search engine optimisation and are also useful in seeing what sort of things are popular on your site. If you have a list of known backlinks that you want to keep track of then you can do it manually, or you can get a script to do it for you.
The following function takes two arguments, the first being the remote URL and the second is the URL you are checking for. The function works by doing a small amount of initial formatting on the URL you are checking for and then downloading the remote page in little bits and seeing if each bit contains a link. If it does then the function breaks out and returns true. If the link isn't there then it returns false.