GOOGLE[
^] is your friend. Visit him often.
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[Edit]
I would suggest doing exactly what you are suggesting. Tie into paypal for payments. Don't maintain any customer financial information on your own site because the liability for that is too great. Let the experts at paypal and such deal with that end. I know this is another google link, but there is some really good information on integrating paypal into php systems there.
php paypal[
^]