There are a number of good answers here on general programming. If you want your knowledge to be relevant to your startup, it might depend on what kind of start

There are a number of good answers here on general programming. If you want your knowledge to be relevant to your startup, it might depend on what kind of startup you're doing. Will it be mobile focused or Web, for example? If web, you might go after JavaScript and some back end language (you already have some PHP background), plus HTML and CSS. Build a simple Web site. If mobile, pick your poison. Many mobile startups start with iOS. Now is a great time to learn iOS since Swift lowers the barrier to entry significantly. If you're going after the larger Android ecosystem, then Java. Android also has the advantage that you can develop on other OS'es like Windows. I would also recommend getting a great book and working though the examples. Then supplement with what you can find on the Web. Good luck learning and with your startup!