Web Page link doesn't work if entered web page doesn't have "http://"

When a user enters a webpage into their profile, if the web page entered is www.yahoo.com, the link will look like this (http://www.webpage.com/user/yahoo.com) if the entire webpage url is entered (http://www.yahoo.com) the link will show and work correctly (http://www.yahoo.com/) There needs to be a url syntax checker. Adding http:// and www when needed, otherwise don't make in hyperlinked.