PHP code to display a PDF attachment

PHP is really not my cup of tea but I prob. can give you some direction to think about:

-Have you get your PHP Toolkit for Salesforce installed and running?  that'd be the first step to get this work.

-Once you have that ready, try doing something like
$conn = new SforceEnterpriseClient();
$conn->createConnection(BASEDIR.'/enterprise.wsdl.xml');
$mylogin = $conn->login(username@domain.com", "changeme");
$query = 'SELECT Name from Account limit 5';
$response = $conn->query(($query));

-Next step is to look at what specific API call is needed to access attachment.  I expect it to be something returning an object or base64 binary string of some sort.  I have no idea on the details as I never looked.  But you should be able to find it in the force.com API docs.

Hope this helps.