Hi,
that's a warning that on a development system you can safely ignore. Apache can't determine the hostname based on the ip address and you didn't specify a hostname in httpd.conf.
That message doesn't have anything to do with possible php problems.
How did you install PHP ? Check the apache error_log file if the apache startup line contains some php info (or error message regarding php).
The create a php file (e.g. info.php) in the htdocs directory that contains
<?PHP
phpinfo();
?>
Then open http://localhost/info.php in your browser. You should then see the phpinfo output.
Thomas