I'd really like to start including shopping cart functionality in my projects.

At first im content relying on paypal links, but I really want to be learning about specific security threats and how to combat them. Eventually I want to feel comfortable receiving and sending customer credit card details for ecommerce.

Obviously this is a common thing on the net but most tutorials and resources are content to say "it's every web developers responsibility to consider security, but we're not going to cover that here/today/ever."

so, my question is, where is a good place to learn? And once I've learned, how do I stay abreast of new vulnerabilities as the web evolves?