PHP download created on the fly contains the source page's HTML

I have a page that gives users the option of downloading some data (in JSON format, if that matters). I am making use of the PHP header command as shown below:

<html>
   <body>
 
<?php 
   $content =  '{"employee": {"name":"bill","salary":36000,"married":true}}';
   header("Content-type: application/json");
   header("Content-Disposition: attachment; filename=savethis.json");
   header("Content-Length: " . filesize($content));
   header("Connection: close");
   print $content;
   exit(0);
?>

   </body>
</html>

But the file that downloads looks like this:

<html>
   <body>

{"employee": {"name":"bill","salary":36000,"married":true}}

Close, but no cigar. What I can do to get rid of the unwanted HTML?