Like other have said it really depends on what you're trying to accomplish. That said, I often find PHP and MongoDB (or any other NoSQL database with good PHP support) a good fit for rapid development on a budget. Those technologies just work really really well in the startup space. Node.js also works and fits just as well as PHP.
The reason why I'm even mentioning some languages and databases here (without any known requirements) is simply for cost and speed.
NoSQL database make it easy to change your mind about schema (not that you shouldn't be paying attention to it). That's important to a prototype and an MVP.
PHP or Node.js (with a good framework) make it easy and FAST to build out your MVP and actually will scale. They are not "just" for prototyping needs. They're very capable languages.
There's two important thing to keep in mind here with PHP/Node.js:
1. Easy to code means lower learning curve and the likelihood of finding a bunch of developers who can built it for you (and that gives you a good range in terms of salary/contract rate)
2. TONS of open-source libraries/modules for you to leverage. The less wheel re-inventing, the faster things come together. You also benefit from open-source in that you have many eyeballs on any given project to help catch bugs, security issues, etc.
Again if you can even use PHP or Node.js or MongoDB is completely up to your application's requirements...But from a business point of view, you may want to be looking at things in this manner (not that PHP and Node.js and MongoDB are your only options either by the way).