What are the potential issues with using the include() function in .tpl files in PHP?

Using the include() function in .tpl files in PHP can potentially lead to security vulnerabilities such as remote code execution if user input is not properly sanitized. To mitigate this risk, it is recommended to use the include_once() function instead, as it ensures that the included file is only included once to avoid conflicts or security issues.

include_once('path/to/file.php');