I don't mean to belittle you or anything like that, but isn't it your job to consult the client on the best tool for the project? If they are adamant on using PHP, when it's clear PHP isn't suited, perhaps that implied an awkward client.
Of course there may be an ideal platform from a technical sense. I may have a project and I decide that it's 100% ideally suited for Scala. Using PHP may be only 85% ideal. However, is that really best for the client? Are they going to be forced to blaze new trails for things that are already solved in PHP? Is the client going to be able to find Scala developers when we leave? How about in 5 years? How much are they going to pay for that? Is it worth the 15% of efficiency that was gained?
There is no real correct answer here, but I guess my point is that when you are talking about what's "best" for the client, it's not always just a technical question.