It doesn't really matter about the format - it depends on the content and if you are targeting Google in particular be aware Google pays most attention to your url and backlinks to your site. Don't pay for links from link farms as that is very outdated and frowned upon and don't indulge in 'I link to you, you link to me' offers as those links count for nothing.
HTML is a client-side markup language. Everything on the web uses HTML. PHP is a server-side language that lets you send different content to different visitors and make a website interactive. It outputs HTML.