Differentiate require and include?

Both require and include are used for file inclusion, but they handle errors differently.

Key Differences:

Practical Examples:

// include - script continues even if file not found
include "optional_config.php";
echo "This will execute even if file missing";

// require - script stops if file not found  
require "database_config.php";
echo "This won't execute if file missing";

// include_once - includes file only once
include_once "functions.php";
include_once "functions.php"; // Won't include again

// require_once - requires file only once
require_once "constants.php";
require_once "constants.php"; // Won't require again

Best Practices:

// Use require for critical files
require_once "config/database.php";
require_once "classes/User.php";

// Use include for optional files
include "templates/header.php";
include "widgets/sidebar.php";

// Error handling with include
if (!include "optional_module.php") {
    echo "Module not available, using default";
}

Performance: require_once and include_once are slightly slower due to file tracking overhead.