hmmm... how to post an image?

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Can't see the photo but here are a few suggestions. First I would repot them into either a bark or coconut based mixture. Moss it difficult for most new orchid growers. Try to make sure you pot into an orchid pot or any other clay pot so the roots can breath and mixture dry out. Finally, remember to always underpot! The roots will appreciate being able to dry out fairly quickly.

Your plant looks fine & healthy! Bottom leaves yellowing & dropping is part of the normal growth cycle. The moss looks old. If it is no longer blooming, now would be a good time to repot. I agree with Smwboxer's advice on repotting, but if you use bark be sure & soak it for 24hrs before doing the repot. The coconut mix would be better if you can get some, fresh spag would also work.

Thanks so much smwboxer and granniek!

I am really worried because the fate of my orchids usually starts with the bottom leaves yellowing (you can see the right bottom leaf is droopy and wrinkly, too), then one by one they lose all the leaves. :(

I will try to repot in bark. I have done that before but have never been able to resurrect any orchid.

To be perfectly honest, it doesn't look fine to me. It doesn't look terrible, but your plant has an overall droopiness to it that suggests something's wrong, and all the leaves are starting to look a bit wrinkly. Seeing as how the roots on top look good, I suspect something going on underneath, in the medium.

I recognize this because Phals and I don't always get along. Especially Phals in sphag. I've done ok with bark and other orchids, but I'm not fond of it for Phals either. I also agree with the recommendation of coconut fiber, either as coco husk chips, or as that coco fiber reptile bedding stuff mixed with bark. I like it because even when wet, it doesn't seem to compact very much, but stays light and airy feeling.

I wouldn't worry overmuch about that yellow leaf. That does look normal to me, especially since there is a root growing right above it. And again, it doesn't look terrible but does look like something's starting to go wrong. I suspect some root rot, and you'll know when you repot. Whichever medium you decide to go with, I recommend the skewer method: stick a bamboo/wooden skewer into the medium, not along the edges but somewhere towards the middle. When you are thinking of watering, pull it out and feel whether or not it's damp. If it is, wait a couple days and test it again. If it's dry, it's ready to be watered. This should help you figure out how often to water.

Ditto what quinnfyre says about the skewer method. Absolutely foolproof. Cut the skewer down to about an inch or so over the medium. Leave it in the medium until you think it's time to water. Touch it to your lip, cheek or the back of your hand. If it's wet or damp, don't water. If it's barely damp or dry, water. How wet or dry the skewer is before the plant needs water will depend on the type of orchid. For example a cattleya likes to dry out completely before being watered again, so the skewer should be very dry. Phals like to almost dry out before being watered again.