basic mcrypt questions

I'm working on my first shopping cart web application (done in php) and have started to look into ways to secure it. After reading up a little bit on mcrypt I have some basic questions about it: 1. If I use mcrypt to encrypt user input before storing it in a database, do I have to use it in conjunction with a utility like PGP, or is mcrypt a self-contained encryption and decryption tool. 2. One article I read almost immediately started talking about public and private keys, another did not mention them at all. Does mcrypt require the use of 3rd party keys? 3. If php scripts are used to decrypt data, where are these scripts kept? Thanks, SW