Why are the leaves on my orchid turning yellow?

Dropping lower leaves can be totally normal as the plant matures, but if it's happening fast it's usually overwatering - phals hate sitting in wet roots and that's the number one killer I see. Check if the potting medium feels soggy and make sure water's draining properly, because root rot will yellow those leaves pretty quick.

The overwatering thing is definitely worth checking first. I learned this the hard way with my orchids actually - I used to think they needed regular watering like my other plants, but they really don't. With phals especially, you want the medium to dry out almost completely between waterings. I water mine maybe once a week or sometimes less depending on how fast the bark breaks down. If you're watering on a schedule rather than checking the roots, that's probably your culprit. Stick your finger in there and see if it feels soggy, and check if there's any smell - that musty, rotten smell means the roots are suffering.

The other thing to consider is your humidity and air circulation. I keep my orchids near a window where they get some air movement, and honestly that's helped them stay healthier than when I had them sitting in stagnant spots. If the lower leaves are just yellowing and dropping gradually, that can be totally normal - orchids do shed older leaves as they grow new ones. But if it's happening really fast or the leaves are mushy rather than just turning color, something's off.

One more thing - check under the leaves for pests or fungal issues. I've only had that happen once with mine, but it can cause yellowing too. Usually though with phals it comes down to either root rot from moisture or sometimes a nutrient thing if you haven't fertilized in a while. Since you've had it eight months, when's the last time you fed it? During growing season I give mine a weak fertilizer every couple weeks or so. Give those roots a chance to dry out and see if it improves over the next few weeks.