What are the potential consequences of using a frame to embed an external site in PHP?

Using a frame to embed an external site in PHP can lead to security vulnerabilities such as clickjacking, where an attacker can trick users into clicking on malicious content unknowingly. To prevent this, it is recommended to use the X-Frame-Options header to control how your site can be embedded in frames on other sites.

header('X-Frame-Options: DENY');