You need to put in all the header stuff for a page. You can't start a page out with the basic html stuff, you need to start with stuff like in the source code for this page:
HTML Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" dir="ltr" lang="en">
<head>
<title>whatever</title>
</head>
<body>
I do not get the error you are getting at all when I upload your page to my test site. I have no file called linkchecker.php called out in the file you posted either.