In this recent post to his blog Vinu Thomas touches on a few wrong ideas floating around about web application security (some specific to PHP) and tries to correct them.
When I talk to developers about security in web development, I usually get the answer that the security is taken care by the systems team by securing the server and by using the https protocol. In reality that is just the tip of the iceberg on security. There's much more you should do as a developer to incorporate security into your applications.
He points out that https doesn't secure your website, it only secures the communication between the client and server. You're still open to all of the usual attacks. Input validation can go a long way to helping to prevent this. He also mentions two insecure practices he's seen over and over - using file names in the URL parameters and using remote includes in an application (giving the remote site a direct line into your application's backend).