  In a comprehensive study, we investigate the electronic scattering effects in
EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ by using Fourier-transform infrared
spectroscopy. In spite of the fact that Eu$^{2+}$ local moments order around
$T_\text{Eu} \approx 20$\,K, the overall optical response is strikingly similar
to the one of the well-known Ba-122 pnictides. The main difference lies within
the suppression of the lower spin-density-wave gap feature. By analysing our
spectra with a multi-component model, we find that the high-energy feature
around 0.7\,eV -- often associated with Hund's rule coupling -- is highly
sensitive to the spin-density-wave ordering, this further confirms its direct
relationship to the dynamics of itinerant carriers. The same model is also used
to investigate the in-plane anisotropy of magnetically detwinned
EuFe$_{2}$As$_{2}$ in the antiferromagnetically ordered state, yielding a
higher Drude weight and lower scattering rate along the crystallographic
$a$-axis. Finally, we analyse the development of the room temperature spectra
with isovalent phosphor substitution and highlight changes in the scattering
rate of hole-like carriers induced by a Lifshitz transition.
