There is a difference between your product being open source versus the tech stack you use to build it being open source. You use email to conduct your business, and that's based on an open standard!

Whoever told you it's "absolutely imperative" that you use Rails instead of an "open" language like PHP ... run as far away from working with that investor as possible.

Just because your programming language is open source doesn't mean acquirers will value you less.

The only way this would come into play is for practical matters. The largest possible one being that the acquirer is on Rails and you're on PHP, so they are less excited because they'll need to rewrite your stuff from PHP to Rails or now also have PHP in their toolkit.

Another minor possible consideration is that open source stacks can sometimes be more exploitable (i.e. security risks), but that's unlikely to affect acquirers.