Another Header headed nowhere

What I want to do is to open a new browser page after some data is entered in a form. I have a $php_self that enables $high_value and $low_value to become set and the code snip comes after the end of the form. From reading a help book and looking at the on-line manual, I think I need to send a header() command (this might not be true, if there is another way, I'll try it too).The following code snip is several lines below the top and it gives me the following error message: Warning: Cannot add header information - headers already sent by (output started at /www/serviceprovider/directory/page3.php:4) in /www/serviceprovider/directory/page3.php on line 364 code snip if (isset($high_value) and isset($low_value)) { header("location: http://www.serviceprovider.com/directory/page4.php"); / / this is line 364 exit; } Your help and advice on this will be greatly appreciated. Hugh