HTTP authentication with PHP

The HTTP Authentication hooks in PHP are only available when it is running as an Apache module. In an Apache module PHP script, it is possible to use the Header() function to send an "Authentication Required" message to the client browser causing it to pop up a Username/Password input window. Once the user has filled in a username and a password, the URL containing the PHP script will be called again with the variables, $PHP_AUTH_USER, $PHP_AUTH_PW and $PHP_AUTH_TYPE set to the user name, password and authentication type respectively. Only "Basic" authentication is supported at this point.

An example script fragment which would force client authentication on a page would be the following:

Example 2-1. HTTP Authentication example <?php if(!isset($PHP_AUTH_USER)) { Header("WWW-authenticate: basic realm=\"My Realm\""); Header("HTTP/1.0 401 Unauthorized"); echo "Text to send if user hits Cancel button\n"; exit; } else { echo "Hello $PHP_AUTH_USER.<P>"; echo "You entered $PHP_AUTH_PW as your password.<P>"; } ?>

Instead of simply printing out the $PHP_AUTH_USER and $PHP_AUTH_PW, you would probably want to check the username and password for validity. Perhaps by sending a query to a database, or by looking up the user in a dbm file.

Watch out for buggy Internet Explorer browsers out there. They seem very picky about the order of the headers. Sending the WWW-authenticate header before the HTTP/1.0 401 header seems to do the trick for now.

In order to prevent someone from writing a script which reveals the password for a page that was authenticated through a traditional external mechanism, the PHP_AUTH variables will not be set if external authentication is enabled for that particular page.

Note, however, that the above does not prevent someone who controls a non-authenticated URL from stealing passwords from authenticated URLs on the same server.