We report oil a recent model introduced to describe the behaviour Of light in a nonlinear active random medium, i.e., laser. The emitted radiation is described by I disordered mean-field Hamiltonian for the phases of the complex amplitudes of the electromagnetic field modes. Using the replica trick the thermodynamics can be exactly solved. A replica-symmetry breaking phase transition is predicted, corresponding to mode-locking processes ill random lasers. The expected random laser phase diagram varying the pumping rate is reported, together with some perspectives opened by the present study.