  The phases as well as the amplitudes of baryonic acoustic oscillations can be
extracted out of a galaxy power spectrum and used as a standard ruler in a
cosmological test. A non-oscillating phenomenological fitting function is used
to extract the oscillatory part of the galaxy power spectrum and to disentangle
phase information from amplitude information. The method (FITEX) is tested with
simulated data of the Hubble Volume Simulation to include redshift space
effects, non-linear structure growth and biasing. A cosmological test is
introduced, which compares the extracted oscillations against a theoretical
model template to derive constraints on the $w$ parameter.
  The phenomenological fitting function is found to model the various
distortions of the galaxy power spectrum to the sub-percent level. The various
distortions only boost the amplitude of the oscillations. The theoretical
template is accurate enough to test for small deviations in the phases of the
oscillations, resulting from different $w$ values. A cosmological test, using
the baryonic acoustic oscillations as a standard ruler, is able to constrain
$w$ in a robust way.
