Hey Clarco,
What Lisa said isn't "entirely" true.
You can use PHP with Site Build It! . They have a feature called Infin It! which allows you to plug in basically anything by "mapping" a third-party provider such as a blog, forums, or in your case, a classifieds site into your existing domain.
The only issue is it would have to be a subdomain. This might not be a bad idea considering that SBI! is really good at bringing traffic. You could use the main part of your website to drive traffic. Then, visitors would naturally go to the PHP part of your site.
You would need to register at a regular host like Lisa suggested, in addition to your SBI! site. But when you "mapped" everything together, search engines would treat it as if it were apart of your site.
To summarize, don't count SBI! out if you are wanting to do something PHP related!

It CAN be done. It just takes a little tweak in the settings (very simple to do) with Infin It! and you can plug in an "infinite" number of things.