How can the use of require/include in PHP files impact the overall functionality of the code?

Using require/include in PHP files can impact the overall functionality of the code by causing fatal errors if the required file is not found or if there are multiple includes of the same file. To solve this issue, it's recommended to use require_once/include_once instead, which ensures that the file is included only once to prevent any conflicts.

// Using require_once to include a file
require_once('path/to/file.php');