How can including a file multiple times lead to conflicts in PHP functions and declarations?

Including a file multiple times in PHP can lead to conflicts because functions and declarations defined in the included file may be redefined, causing PHP to throw errors. To avoid this issue, you can use the `include_once` or `require_once` functions instead of `include` or `require` to ensure that the file is only included once.

// Using require_once to include a file only once
require_once 'file.php';

// Now you can safely call functions or use declarations from the included file without conflicts