<?php
  mysql_connect("your.database.host",
                "your-username", "your-password")
    or die("<p>Error connecting to database: " .
           mysql_error() . "</p>");

  echo "<p>Connected to MySQL!</p>";

  mysql_select_db("your-database-name")
    or die("<p>Error selecting the database bmclaugh: " .
           mysql_error() . "</p>");

  echo "<p>Connected to MySQL, using database bmclaugh.</p>";

  // And so on...
?>

So suppose you want your code to look more like this (actually, you really do want it to look more like this):

<?php
  mysql_connect($database_host, $username, $password)
    or die("<p>Error connecting to database: " .
           mysql_error() . "</p>");

  echo "<p>Connected to MySQL!</p>";

  mysql_select_db($database_name)
    or die("<p>Error selecting the database bmclaugh: " .
           mysql_error() . "</p>");

  echo "<p>Connected to MySQL, using database bmclaugh.</p>";

  // And so on...
?>

All you’re doing is writing something that looks a bit like a variable in place of hand-typing the username or database name. Now you can define those variables up above your connection code:

<?php
  $database_host = "your.database.host";
  $username = "your-username";
  $password = "your-password";
  $database_name = "your-database-name";

  // Database connection code
?>

But is this really that much better? Not yet; you’ve still got these same values hand-typed into your script. You want to stick the values in a file so no human has to type them. Read on.

Your goal is to get these values out of connect.php, into some place that all your PHP scripts can access them with no typing from you. Open up a new file, and call it app_config.php. Now drop your variables into this new file:

<?php
// Database connection constants
$database_host = "your.database.host";
$username = "your-username";
$password = "your-password";
$database_name = "your-database-name";

?>

When you move to a production version of your application, you probably want to place this file outside of the site root. That way, web users can’t simply type the path to your configuration script and get all your passwords. Alternatively, you could add security to this directory, although simply getting it out of the web-serving directories altogether is usually easiest.

Now, you can have all your different PHP scripts use these shared variables. Change a variable here in app_config.php, and that change affects all your PHP scripts that use these shared variables.

But how do you actually access these variables? Go back to connect.php, and remove where you defined these variables yourself. If you try and access connect.php through connect.html now, though, you’ll get a nasty error, as shown in see Figure 4-6.

The error occurs because connect.php now has no idea what $username or $password refers to. You need to inform PHP that before it tries to do anything in connect.php, it’s required to load app_config.php. And that’s (almost) exactly what you type in your script:

<?php

  require '../../scripts/app_config.php';

  // Database connection code
?>

Now, PHP loads the file ../../scripts/app_config.php before it runs your mysql_connect function. In effect, require says, “Hey PHP, if you can’t load the file I’m giving, then throw a nasty error, because nothing else is going to work.”

Try and run your connection script again, and you should see your table listing, which means things are working well again.

There’s just one more nagging little problem with your code: you’re still using variables for your username and password, along with the database host and database name. And what’s a variable? Something that varies or changes. Accordingly, PHP happily lets you write the following code in connect.php:

mysql_connect($database_host, $username, $password)
  or die("<p>Error connecting to database: " . mysql_error() . "</p>");

// This is allowed, but some bad mojo
$password = "hijinks";

So what happens when some other script—which also requires app_config.php—tries to connect with mysql_connect? It’s going to use $password, but now $password isn’t correct anymore. It’s set to “hijinks,” and chaos will ensue.

What you really want is for those values in app_config.php to be constant, and never change. You can do this with the special define function. Open up app_config.php and change your code:

<?php
// Database connection constants
define("DATABASE_HOST", "your.database.host");
define("DATABASE_USERNAME", "your-username");
define("DATABASE_PASSWORD", "your-password");
define("DATABASE_NAME", "your-database-name");
?>

You define the name of a constant and the value for that constant, and PHP creates a new constant. That way, you can type DATABASE_HOST into your code, and PHP really sees “your.database.host”. Perfect! And since this is a constant, it can’t be changed anywhere along the line.

Constants are typed in all-uppercase letters. Caps aren’t required, but it’s another one of those “speak like a PHP programmer” things. You want constants to look different from variables, and using all uppercase names is one way to do that. Constants also don’t have the $ before their name, which is yet another way to differentiate a constant from a variable.

Now you need to make some quick changes to connect.php to use these new capitalized names of constants:

<?php
  require '../../scripts/app_config.php';

  mysql_connect(DATABASE_HOST, DATABASE_USERNAME, DATABASE_PASSWORD)
    or die("<p>Error connecting to database: " .
           mysql_error() . "</p>");

  echo "<p>Connected to MySQL!</p>";

  mysql_select_db(DATABASE_NAME)
    or die("<p>Error selecting the database " . DATABASE_NAME .
           mysql_error() . "</p>");

  echo "<p>Connected to MySQL, using database " . DATABASE_NAME . "</p>";

  // SQL query-running goodness proceeds...
?>

Try connect.php again. You should get a perfectly good list of table names. But this time, you’ve got constants for your important information, safely tucked away in a file separated out of connect.php.