I am sure that the people putting on the play have already paid some kind of fee to put on the play. The play's copyright covers the words in the play and usually not the visuals, which change from production to production through the directors'/producers' interpretation.
If the people putting on the play have asked you to photograph the production for publicity purposes, I will assume that they are responsible for copyright issues.
Unless you are recording (audio) the words or photographing the text, I would think you are all set. But in this age of over-litigation, do some investigating.
I have photographed local theatrical productions in the past and have had no problems.
good luck!