PHP is still relevant. However coding like its 1999 is not relevant. Keep it fresh - adopt modern development patterns suitable for your project, be PSR-2 compliant, use the latest version of PHP etc.

If you're using Mandrill, this is a good time to have a look at MailGun. https://www.mailgun.com

To become a CTO you need to have a broad understanding of the "T"-part, the broader - the better. As for "C", besides from the generics (mentoring, leadership, planning, methods etc) tech-people have specific needs that only tech-people understand. Get good at understanding them. And for the "O"-part, you must have the ability to communicate ideas and "T" in such a way that non-tech people understand.

There are many ways to make money as a developer. Here are 4 examples: ThemeForest - create themes in bootstrap, or for Wordpress and sell them here Envato - write tutorials and get paid oDesk, UpWork etc. - find a side-gig and work it Write an app

Go is pretty awesome for building API's. I've ported an API from PHP to Go - code reduction is more than 50% and performance is just not comparable (in favor for Go).