Yes, I have, and the client was right to raise these concerns.
There's a popular truism that "Clients don't care about language, clients care about solutions". There's a big truth in there, in that ultimately a business wants to spend n number of dollars because it's going to get n * x number of dollars back.
However, a smart client (or a smart executive) cares about the implications of the solution they're buying, and they care about the platform. It is easier to build trivial, non-robust solutions in PHP. In aggregate, PHP developers are less experienced software engineers than PHP developers. Lacking other knowledge, that's going to be the baseline when you're trying to sell any PHP based service.
This doesn't mean you can't land these gigs, but you need to make it clear to the smart executives that your solution overcomes PHP's problems. Find a way to demonstrate to them you have experienced developers on staff. Find the weaknesses in your open-source platform caused by PHP's architecture choices and explain how you've overcomes them and brought reliable software engineering to a platform that's more known for it's cowboy code monkey antics.
You're also going to run into dumb clients, (or dumb executives) who don't care about the solution they're buying, and don't care about the platform. These executive will know what the common prejudices are, and will press you on them. The flip side of "No one was ever fired for buying IBM Microsoft".
BOTH smart clients and dumb clients will know the going rate for an average PHP developer. All other things being equal, you'll have an easier time billing out at higher rates if you're a Java or .NET shop. Also, clients willing to shell out for the .NET licensing fees are going to (again, in aggregate) be more willing, in general, to pay higher rates for development.
So if you have a PHP solution you need to make it clear that all things aren't equal. You need to make it clear that PHP makes you a better choice, and is going to help that client (or the individual executive) turn n dollars into n times x dollars.