What are alternative methods to require_once that can be used in PHP to avoid variable handling issues?

When using `require_once` in PHP, variable handling issues can arise if the included file declares variables that are already in use in the parent file. This can lead to conflicts and unexpected behavior. To avoid these issues, alternative methods such as `include_once`, `require`, or using namespaces can be used to include files without causing variable conflicts.

// Using include_once to include the file without causing variable conflicts
include_once 'file.php';