Yellowing bottom leaves on Phal

[hatter]

Hi Everyone

I got engaged about 2 months ago and my mother-in-law to be bought be a beautiful white phal as a congratulatory present. I used to have 3 orchids a few years ago and lost them all to what I can only assume was a virus (being a beginner I didnt know to keep the new plants away from the old ones and they all got sick)

Anyway, I don't want this one to die too - especially because it has sentimental value.

When I got it, I noticed it had one slightly yellow bottom leaf, but assumed this might just be an old leaf. Now a leaf higher up has gone yellow and looks like its rotting. It has taken about 6 weeks for this leaf to go from healthy to the state it is in now (see photos).

The original yellow leaf is still there, and the other bottom leaf is also a bit droopy.

Any advice appreciated.