  The $\phi$-meson production cross section is measured for the first time at a
sub-threshold energy of 1.93 AGeV in \nuc{58}{Ni}+\nuc{58}{Ni} central
collisions. The $\phi$ data were obtained within the acceptance of the
CDC/Barrel subsystem of FOPI. For a sample of $4.7\cdot 10^6$ central events,
after background subtraction, 23 candidates were observed. Extensive GEANT
simulations of the detector performance are shown in a thorough comparison to
the real response, aiming at a good understanding of the apparatus and at a
trustable determination of the efficiencies, production probability and
possible systematic errors. A filter procedure is elaborated, which is meant to
facilitate the comparison of any theoretical calculation or new data with the
current ones. How to extrapolate the present value to a $\phi$-meson cross
section in $4\pi$ is also discussed. This result on pseudo-vector mesons can
now be compared to existing experimental knowledge for the same reaction at the
same incident energy for various outgoing channels, $K^+$ and $K^-$ included. A
significant fraction (at least 20%) of the $K^-$-mesons is originating in the
decay of the $\phi$, supporting the statement that the two channels are
strongly correlated.
