Welcome, JAG01010! Soon, you will be an orchid expert.
Sometimes old leaves just turn yellow and fall off. However, for most orchid beginners, it is caused by overwatering, leading to root rotting, leading to the plant not being able to take up enough water.
There is a thread here devoted to teaching beginners (and older growers too, ha ha) how to keep from killing Phals. Here it is:
The Phal abuse ends here.