SSL and PHP really don't have anything to do with one another, there is nothing special you need to do with your PHP code to work with SSL. PHP runs on the server. SSL is a protocol by which data is sent from the client to the server. once the data is on the server then it is handled by PHP normally.
one small gotcha: when working with PHP sessions, be advised that a HTTPS URL is of course considered a different domain then a similar HTTP URL and as such session ID's are not automatically propagated.