Question: My server provides SSL connections via https, although the certificate costs extra...

Is there anything that needs to be changed in the PHP code to utilize this protocol?

My site has:

* ajax forms via POST
* regular forms and pages using POST and GET parameters
* Session variables

Answer: 
You should be good to go. PHP does not impact the use of SSL or not.

Things you should check are:

* Are all URLS in you application relative (no `http://`)
* Are assets (CSS/JS/IMG) used in your site (***both from internal*** and external sources) also as relative paths or prefixed with `https://`

Having an asset without `https://` in a SSL powered site, the browsers will warn you visitors that something ain't right.
you can use the server .htaccess file to redirect all your links. So when the standard page is opened via say a link the server redirects to the https version...

```
# Permanent reirect ALL old pages to HTTPS:
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

```