The book discusses the dynamic parternship between PHP and MySQL and the advantages of each one. PHP is a scripting language that specifically was created for use on the web and allows the creation of dynamic web pages (pages that interact with customers, rather than serve as a static page) and also serves to communicate with the database management system, which MySQL is an example of.
In the relationship, PHP creates a connection with the MySQL server and sends an SQL message through the connection. The MySQL server interprets the message and follows the instructions, and sends a response. If it does not understand the message, an error report comes onto the web browser (I'm sure we can all relate to this part). SQL is the language used to add, delete, and edit information within the database.
In the database, PK stands for "Primary Key", and "FK" stands for foreign key. This example to the left shows how variables interact with one another. A variable may be a Primary Key in one field and a Foreign Key in another.
For example, Contact Name ID may be the Primary Key of the "Contact Name" field, and all the other variables are details describing that particular Customer Name ID. In another field, such as Contact Address, the Contact Name ID is a Foreign Key that is used to represent a description of that field's Primary Key. Because the database is relational, the Contact Address field will thus share all the same information about that Customer Name ID that is described in the Contact Name.
Hopefully this helps shed a little light on databases.