How can PHP developers ensure secure content delivery when embedding external pages using iframes in PHP?

To ensure secure content delivery when embedding external pages using iframes in PHP, developers can use the `Content-Security-Policy` header to restrict the sources from which the iframe can load content. By setting the `frame-ancestors` directive to `self`, developers can ensure that the iframe can only be embedded on pages from the same origin.

<?php
header("Content-Security-Policy: frame-ancestors 'self'");
?>