Sorry I wasn't thinking clearly when I put this up. The error is reported to be in this line:
$data = mysql_fetch_array($query);
Therefore, I believe the problem is in my $query.
The trouble is, I don't know what is at fault in the $query.
This fault does not occur on my browsers, only on others going to the site to login.
All login operations function correctly. Visitors can sometimes remove the fault by refreshing the page, but this doesn't work on all browsers.
Let me know what you think. Thanks,
Kevin