How can PHP developers ensure the security of their applications when including files dynamically based on user input?

When including files dynamically based on user input, PHP developers should validate and sanitize the user input to prevent directory traversal attacks. One way to do this is by using a whitelist approach where only allowed files are included based on predefined rules. Additionally, developers should ensure that the included files are located within a secure directory to prevent unauthorized access to sensitive files.

// Validate and sanitize user input before including files dynamically
$allowed_files = ['file1.php', 'file2.php']; // Define a whitelist of allowed files
$user_input = $_GET['file']; // Get user input
if (in_array($user_input, $allowed_files)) {
    include 'secure_directory/' . $user_input; // Include the file from a secure directory
} else {
    // Handle invalid input
    echo 'Invalid file specified';
}