Are there any specific PHP functions or techniques that can enhance the security of included files, such as using defined constants for validation?

When including files in PHP, it's important to validate the files being included to prevent security vulnerabilities such as directory traversal attacks. One way to enhance the security of included files is to use defined constants for validation. By defining a constant with the absolute path to the directory where the included files are stored, you can ensure that only files within that directory are included.

// Define a constant for the base directory where included files are stored
define('BASE_DIR', '/path/to/included/files/');

// Validate the included file using the defined constant
$included_file = BASE_DIR . 'file-to-include.php';

if (file_exists($included_file)) {
    include $included_file;
} else {
    // Handle error or log unauthorized access
}