But what would you say to this person?
Me? Probably nothing. Those who've gone down the path of buying hype aren't likely to listen to reason. Now if the manager had said "sorry, we've evaluated Perl but PHP better fit our needs", I'd be more likely to agree with this person. Sometimes PHP is a better fit.
If you have reason to believe that he's simply chosen a technology without clearly defining his needs, you could forward him this essay I wrote recently. At first it seems to only be peripherally related, but at the bottom I address that point. However, as I said at first, once people start to believe hype, they're not likely to stop. In fact, arguing with them may make your case worse. If you already know PHP, why not use that language?