What are common reasons for the error "No such file or directory" when using PHP with external libraries like Smarty?

The error "No such file or directory" typically occurs when PHP cannot locate the necessary files or directories for external libraries like Smarty. This could be due to incorrect file paths or missing files. To solve this issue, double-check the file paths in your PHP code and ensure that all required files are present in the correct directories.

require_once('path/to/Smarty/Smarty.class.php');