How can one pass variables to included files in PHP?

To pass variables to included files in PHP, you can use the `include` or `require` function along with the `$_GET`, `$_POST`, or `$_SESSION` superglobals to pass variables to the included file. You can also define variables before including the file and access them within the included file.

// Define variable
$variable = "Hello World";

// Include file and pass variable
include 'included_file.php';

// Access variable in included file
echo $variable;
```

In the included_file.php:
```php
// Access variable passed from main file
echo $variable;