Talking about Struggling
Absolutely do talk about your difficulties. Have some specifics in mind on what might help, and be open to feedback. The thing to avoid is be so general that there's no way to get you help. "I don't know, it's just really hard" won't help - throughout your career, you are likely to tackle problems that seem impossible. The trick is to build up a set of tricks for finding help.
Have some details - what's been difficult to do? What's been pretty easy? Would having a specific person available to ask questions to help? What learning resources have you tried and are others available?
A big one that I see when people transition in these areas is that they don't realize where and how to diagnose issues - the debug process in a .NET server is different than in PHP... so often it's just not knowing the ways to debug that hang people up.
Having a list of what you've tried and how it's worked is helpful. You don't need a two page monologue, but you want your thoughts in some sort of an order so you can have a meaningful discussion.
Asking for New Work
This one's a bit iffy. If someone came to me and said "what you asked me to do is really hard. So I want to do something else" --- the conversation would not go well. Engineering salaries are relatively high and I expect that people can take on hard work and complete it. As an engineering manager, I don't have the luxury of making work available in the languages people are most comfortable with, I have to move my team to where the work is. Having an engineer that tells me that they simply can't learn the new language will make me wonder about the long term viability of the engineer.
If you love PHP and you know of a project that needs help that uses PHP, you may be able to say - "hey, this project sounds fascinating, what's the chances I get into that group?" - that makes a much better case than just not feeling able to tackle ASP. If you have a career goal of becoming a PHP guru, that may be another way to phrase the request - "I love PHP so much, I'm really eager to pick up more work in it" - but you may have to be ready to hear that the boss simply doesn't have more work available.
If what you're hearing or seeing is that there isn't work of the type you like doing in this company, then it's time to look elsewhere. Personally, I'd advocate learning the new stuff rather than job hunting, because my experience in the web world has been that getting a broad base in web development techniques is more useful than digging down into a specific implementation mechanism, but it's really your call. I have respect for those who have stuck with one language and really mastered it - the risk is always that staying in one language means you are just as marketable as that one language - so if it goes out of vogue, so may your career if you haven't proved you're good at learning new things.