  We present a new synthetic model to follow the evolution of a planetary
nebula (PN) and its central star, starting from the onset of AGB phase up to
the white dwarf cooling sequence. The model suitably combines various
analytical prescriptions to account for different (but inter-related) aspects
of planetary nebulae, such as: the dynamical evolution of the primary shell and
surrounding ejecta, the photoionisation of H and He by the central star, the
nebular emission of a few relevant optical lines (e.g. Hbeta; HeII4686;
[OIII]5007). Predictions of the synthetic model are tested by comparison with
both findings of hydrodynamical calculations, and observations of Galactic PNe.
The sensitiveness of the results to the models parameters (e.g. transition
time, mass of the central star, H-/He-burning tracks, etc.) is also discussed.
We briefly illustrate the systematic differences that are expected in the
luminosities and lifetimes of PNe with either H- or He-burning central stars,
which result in different ``detection probabilities'' across the H-R diagram,
in both Hbeta and [OIII]5007 lines. Adopting reasonable values of the model
parameters, we are able to reproduce, in a satisfactory way, many general
properties of PNe, like the ionised mass--nebular radius relationship, the
trends of a few main nebular line ratios, and the observed ranges of nebular
shell thicknesses, electron densities, and expansion velocities. The models
naturally predict also the possible transitions from optically-thick to
optically-thin configurations (and vice versa). In this context, the origin of
the Zanstra discrepancy is also analysed. (abridged)
