  Inflation is the leading theory to describe elegantly the initial conditions
that led to structure formation in our universe. In this paper, we present a
novel phenomenological fit to the Planck, WMAP polarisation (WP) and the BICEP2
datasets using an alternative parameterisation. Instead of starting from
inflationary potentials and computing the inflationary observables, we use a
phenomenological parameterisation due to Mukhanov, describing inflation by an
effective equation-of-state, in terms of the number of e-folds and two
phenomenological parameters $\alpha$ and $\beta$. Within such a
parametrisation, which captures the different inflationary models in a
model-independent way, the values of the scalar spectral index $n_s$, its
running and the tensor-to-scalar ratio $r$ are predicted, given a set of
parameters $(\alpha,\beta)$. We perform a Markov Chain Monte Carlo analysis of
these parameters, and we show that the combined analysis of Planck and WP data
favours the Starobinsky and Higgs inflation scenarios. Assuming that the BICEP2
signal is not entirely due to foregrounds, the addition of this last data set
prefers instead the $\phi^2$ chaotic models. The constraint we get from Planck
and WP data alone on the derived tensor-to-scalar ratio is $r<0.18$ at
$95\%$~CL, value which is consistent with the one quoted from the BICEP2
collaboration analysis, $r = 0.16^{+0-06}_{-0.05}$, after foreground
subtraction. This is not necessarily at odds with the $2\sigma$ tension found
between Planck and BICEP2 measurements when analysing data in terms of the
usual $n_s$ and $r$ parameters, given that the parameterisation used here
includes, implicitly, a running spectral index.
