The htdocs directory in an Apache installation, is the directory from which Apache will serve the webpages from. This is probably /path/to/apache/htdocs, or /usr/local/apache/htdocs.
By placing the following text, in a file (preferably called test.php), within the htdocs directory, one can verify the installation of PHP.
<?phpinfo();?>
By viewing the file through Apache, the browser must be pointed to http://127.0.0.1/test.php. One will see the different configuration parameters that were passed to the PHP configure script, as well as many other details.