Interesting - this was in the Hostgator help section and it looks like a MODx issue:
From Hostgator
PHP Cannot modify header information
Warning: Cannot modify header information - headers already sent by (output started at /home/user/public_html/test.php:3) in /home/user/public_html/test.php on line 4
Look familiar? This PHP error has haunted website owners for years. Please understand, HostGator does not normally support your webdesign and coding, but this error is too common for us to ignore. So, one of our PHP experts has the answer for you.
The problem you face is that you are trying to use the PHP header function, header(), but there can be absolutely no HTML output before this function is declared. Let’s use some examples:
<!DOCTYPE "HTML 4.01 Transitional EN" "
http://www.w3.org/">
<html><body>
<?php header (’Location:
http://hostgator.com/’); ?>
</body>
</html>
This is incorrect coding. You cannot have any HTML preceding the header function. A few more bad examples:
<?php
echo "Task complete.";
header (’Location:
http://hostgator.com/’);
?>
Task complete! <?php
header (’Location:
http://hostgator.com/’);
?>
This is incorrect coding for the same reason. You cannot have any text output before the header function. Plain text is always treated as HTML by your browser. One more bad example:
<?php
header (’Location:
http://hostgator.com/’);
?>
This one is tricky. The blank space before the PHP tag counts as HTML output.
It is acceptable to have other PHP code before the header function, as long as nothing is outputted as HTML. This is a correct example:
<?php
$variable = "value";
mysql_query("blah blah");
header (’Location:
http://hostgator.com/’);
?>
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