PHP - What Does Ssl Affect?

I've been coding PHP for a while now and have never worked with e-commerce. Looking into start developing my own e-store. Payment gateways require your site to have SSL. Does SSL affect any of the PHP code? Why doesn't every PHP coded site have SSL if it protects your site? anything i should look out for? I'm not even sure how to implement the SSL but i want to get an idea of what i need to be prepared for. Thanks.