What are the best practices for including external content, such as links, in PHP files to ensure proper functionality and maintainability?

When including external content, such as links, in PHP files, it is important to properly sanitize and validate the input to prevent security vulnerabilities like cross-site scripting (XSS) attacks. One way to do this is by using the htmlspecialchars() function to escape special characters in the input data before outputting it to the browser. This ensures that the content is displayed as intended and not executed as code.

$link = "<a href='http://example.com'>Click here</a>";
echo htmlspecialchars($link);