How can developers ensure that included PHP code remains synchronized and consistent across different files?

To ensure that included PHP code remains synchronized and consistent across different files, developers can use PHP's `require_once` or `include_once` functions. These functions will include a file only once, even if it is referenced multiple times in different files. This helps prevent issues with duplicate declarations or conflicting code.

// Include the file once to ensure synchronization
require_once 'path/to/file.php';