What potential issues can arise when using include_once in PHP?

When using include_once in PHP, potential issues can arise if the included file contains functions or classes that have already been declared in another included file. This can lead to redeclaration errors and cause unexpected behavior in your code. To solve this issue, you can use the require_once function instead, which will include the file only if it has not been included before.

require_once 'file.php';