Growing up in Phoenix, in particular the Arcadia neighborhood, Thai Lahna is sentimentally one of my favorite Thai restaurants in the area. It's genuine, straight-forward and now oft-familiar Thai fare served in a clean, well-maintained atmosphere. It has all of the golden mom-and-pop attributes an enthusiast seeks without the (easily misguided) trappings less-seasoned Thai explorers might find distracting.
Be forewarned, the service can be slow, often frustrating. During peak times, when it's most busy, don't prance in thinking you'll get in and out in a short timeframe. Sometimes, having overly bubbly, cheerful service is, well, not necessarily a priority with the owners. Most of the time, the service at Thai Lahna is perfectly functional, and that's all that matters.
The important thing about Thai Lahna is the food. All of the curries, noodle dishes and soups, are without a doubt baseline good, and worth most superficial hardships you may incur with the previously touched on service. Even if it's not the "best Thai food in the world," everything often tastes like it should.
It's worth a visit if you live or happen to be in the area.