What steps can be taken to ensure that variables are properly passed when including external PHP files in a script?

When including external PHP files in a script, it's important to ensure that variables are properly passed to the included file. One way to do this is by using the `include` or `require` functions along with passing variables as parameters. Another way is to use the `global` keyword to access variables defined outside the included file within the file.

// Using include with passing variables as parameters
$variable = "Hello";
include 'external_file.php';
```

```php
// Using global keyword to access variables defined outside the included file
$variable = "Hello";
function myFunction() {
    global $variable;
    echo $variable;
}
include 'external_file.php';