  In this paper we determine the phase diagrams (for $T=0$ as well as $T>0$) of
the Penson-Kolb-Hubbard model for two dimensional square lattice within
Hartree-Fock mean-field theory focusing on investigation of superconducting
phases and possibility of the occurrence of the phase separation. We obtain
that the phase separation, which is a state of coexistence of two different
superconducting phases (with $s$-wave and $\eta$-wave symmetries), occurs in
define range of the electron concentration. In addition, increasing temperature
can change the symmetry of the superconducting order parameter (from
$\eta$-wave into $s$-wave). The system considered exhibits also an interesting
multicritical behaviour including bicritical points.
