How can PHP developers ensure cross-browser compatibility when using iframes to display external content?

To ensure cross-browser compatibility when using iframes to display external content, PHP developers can set the `sandbox` attribute on the iframe element to restrict certain behaviors like scripts, forms, and pop-ups. This helps prevent potential security risks and ensures consistent rendering across different browsers.

<iframe src="https://example.com" sandbox="allow-same-origin allow-scripts"></iframe>