HTTP AUTH. help!

Hi! I´ve read that http authorization with php only works if I have installed php as an module to apache..!? And I´m not quite sure if I installed it as an module, because I´m a newbie and only followed the instructions on a page I found. (http://www.devshed.com/Server_Side/Administration/Database/print.html) Well, anyways... The script I wrote looks fine, but it doesnt work! I get the following error:

Warning: Cannot add header information - header already sent by (output started at /home/httpd/htdocs/hlbtest/auth_old.php:5) in >home/httpd/htdocs/hlbtest/auth_old.php on line 13.

Warning: Cannot add header information - header already sent by (output started at /home/httpd/htdocs/hlbtest/auth_old.php:5) in >home/httpd/htdocs/hlbtest/auth_old.php on line 14.

And the error refers to : if(!isset($PHP_AUTH_USER)) { header ('WWW-authenticate: Basic realm="HLB"'); <-- line 13 header('HTTP/1.0 401 Unauthorized'); <-- line 14 exit; Can this mean that i´ve not installed php as an module? I downloaded this file PHP-4.0.1pl2.tar.gz, from http://www.php.net. Or can it be that I´ll have to edit some .conf file or something? Hope someone can help, thanks Rikard..