If the photon had mass, then the Faraday's and Ampere equations would pick up an additional term related to the mass of the photon. This term will give rise to the Hall effect exhibited by semiconductors when a magnetic field is applied on. The mass of the photon will accordingly equal to m=(I hbar/Qc^2), where I is the current passing on the sample, Q is the total charge enclosed by the sample, and hbar is the Planck's constant divided by 2pi. Any challenges to measure this?
This can be transformed into a relation m=hbar v/(Wc^2), where v is the electron drift velocity and W is the width of the sample.