Help with PHP and MySQL

I am not sure how to frame my questions so I will just do my best.

I have a table called rooms and it has three fields: roomsP§, roomNumber, imageDate

I am populating a drop down menu with the roomNumber field. What I want to do from here is populate a text field with the image date that is in the same entry as the shown roomNumber,. So, I choose room number 1 and its imageDate appears in the text field.

Any help would be greatly appreciated. Thank you in advance.