Novice in eCommerce

this may not be the right place to ask this, but perhaps someone here can tell me what I don't know and should be trying to learn! I can uses php in the construction of a web site. I can create a data base and authenticate users. I can create a shopping cart. I know that osCommerce is highly thought of. I can get a merchant card. I know that a SSL certificate is needed and if it is shared due to a shared server, the insurance is only for the owner of the server not for those accounts hosted. I know how to encrypt email using PGP or GNUPG. What I seem to be dense about is the technique for connecting the credit card company and the shopping cart, and where the SSL fits into this code! Something just isn't making sense! My appologies if I am trying to get php to do something that it isn't meant to do! If you can tell me what should be doing it I would greatly appreciate it!!!!!!!!! TIA Karen