I need to include an external page into my website. How SEO friendly is this? And which will be the right code to use? I came a accross a few different codes and I'm not sure which to use...
Your question as is does not target a specific problem and no code is provided.
You can fetch anything you want and integrate into your website, using output buffers, file_get_contents(), SOAP, REST or cURL.
Search engines do not recognize the fetched content as external, but your page load time might increase if you do not buffer the content which decreases SEO value.
The better question is, does the site you want to fetch from allow using of their content, what licence does it have?