In PHP, what considerations should be made when embedding images using scripts like "image.php" to fetch image URLs from a database or external source?

When embedding images using scripts like "image.php" to fetch image URLs from a database or external source, it is important to sanitize user input to prevent SQL injection and other security vulnerabilities. Additionally, it is recommended to validate the image URL before processing it to ensure it is a valid image file. Finally, consider implementing caching mechanisms to improve performance by reducing the number of requests to the database or external source.

<?php
// Validate and sanitize input
$image_id = filter_input(INPUT_GET, 'image_id', FILTER_SANITIZE_NUMBER_INT);

// Retrieve image URL from database
// Example: $image_url = fetch_image_url_from_database($image_id);

// Validate image URL
if (filter_var($image_url, FILTER_VALIDATE_URL) && exif_imagetype($image_url)) {
    // Output image
    header('Content-Type: image/jpeg');
    readfile($image_url);
} else {
    // Output default image or error message
    header('Content-Type: image/jpeg');
    readfile('default_image.jpg');
}