  Spin-magnetophonon level splitting in a quantum well made of a semimagnetic
wide gap semiconductor is considered. The semimagnetic semiconductors are
characterized by a large effective $g$ factor. The resonance conditions
$\hbar\omega_{\rm LO}=\mu_BgB$ for the spin flip between two Zeeman levels due
to interaction with longitudinal optical phonons can be achieved sweeping
magnetic field $B$. This condition is studied in quantum wells. It is shown
that it leads to a level splitting that is dependent on the electron-phonon
coupling strength as well as on the spin-orbit interaction in this structure.
  We treat in detail the Rashba model for the spin-orbit interaction assuming
that the quantum well lacks inversion symmetry and briefly discuss other
models. The resonant transmission and reflection of light by the well is
suggested as a suitable experimental probe of the level splitting.
