William Steinmetz - Wicked Cool PHP: Real-World Scripts That Solve Difficult Problems

Most serious applications have a core library of variables and scripts that are used on almost every page. For example, if you're writing a shopping cart that connects to a MySQL database, you could declare the MySQL login name and password on each page of your cart. But what if you need to change the password? Changing and uploading every file in your shopping cart could become a huge issue.

Rather than declaring the password in each of your page scripts, you can store that name and password in a separate file. You can then include that file as a part of your script, and whatever variables you declare in that file will be declared in your script!

Furthermore, you can store long scripts or functions in a separate file and include them only when you need them. For example, the function that gets real-time UPS shipping quotes is 24KB worth of XML processing goodness, but you use it only when someone chooses UPS as a shipping option. Why not store it in ups_ship_quotes.php and call it only when necessary?

In fact, almost all heavy-duty PHP applications have a file called something like config.php, which declares the critical variables that every page needs to know, such as the MySQL name and password. Those same applications also store frequently used scripts in different directories. Programmers then mix and match, taking the check-to-see-if-a-user-is-logged-in script from one directory, including the get-the-relevant-data-from-the-database script from another directory, and writing a central script that screens the data based on whether the user is logged in or not.

And here's how to do it.