PHP has its shortcomings, but, as in any open source product, if it doesn't work for you, you have a couple options.
They can't bully their way like they tried.
The decision was defended on its own merit, so I would be really disappointed if bringing money to the table affected the outcome. Funding open source development is great as long as meritocracy is maintained. That's why Linus never accepted a job at a company that had a stake in pushing Linux in a certain direction.
I personally think this guy should ask for a full money-back refund and then shut the hell up.
I'm of the mindset to stick with what I know best when I'd rather build a working product and get it out the door quickly. I don't actually personally care too much what language I use (I feel like database selection is more crucial) but I read about so many startups running on Python or Rails that I'm starting to wonder if there's something I'm missing and if there are business advantages to using other languages/frameworks.
Contrary to the other comment, there are business advantages to using a particular language. (Though I would concede if your only choices are Ruby's Rails, Python's Django, and PHP's Yii, for many problems there isn't much to compel you to one or the other besides your preference and available talent.) I could write a large comment going over the pros/cons for different use-cases of PHP (plain or with a framework like Yii), Python (with Flask), Java (with enunciate), and Node.JS (those are the only languages/environments I've built larger-than-toy webapps with; I still need an excuse to use Clojure's Noir for something). My list would not just be language-war pros/cons but business value considerations and hypothetical consequences. It's not the most important choice you can make, but it should be considered if there's more than one option because the type of problem you're solving can be made much simpler or easier with the right tool.
I think you've got the right mindset, and that's to use what you know for anything important you need to finish soon, but I'd recommend checking out the other environments on your own time just for fun. Also before starting a project, research to see if it's a solved (or mostly solved) problem for another framework/language. Even if there's a learning cost to something brand new, depending on the problem it can be well worth it for the overall cost reduction that the tool provides. There are a lot of "We learned and used/migrated to Erlang" stories out there because Erlang solves particular problems very well.
I often find myself wanting to start new projects in $new_platform but have no idea how to really compare them before I get started, save for looking at their documentation or advertised features.
You should only use the technology that suits your company the best (in terms of needs and knowledge -never EVER start a company based on a tech you don't understand or know)
Upon reading the conversation I was amazed he didn't get that response right away.
And, to be honest, he is clearly demonstrating the fact that he's a pretty poor developer, and that he doesn't have the necessary qualifications to be writing software that manages people's retirement funds. Also, when dealing with something as important as that, you ought to know better than to base our technology on top of PHP.
[1]: Coming from somebody who built a very successful startup on top of PHP.