  Using Wannier function-based interpolation techniques, we present compelling
numerical evidence for the presence of a saddle-point van Hove singularity at
the $\Gamma $ point near the phosphorene Fermi energy. We show that in
proximity of the van Hove singularity the spin susceptibility presents the
logarithmic temperature dependence typical of Liftshitz phase transitions.
Furthermore, we demonstrate that the critical temperature for the ferromagnetic
transition can be greatly increased (up to 0.05 K) if strain along the zigzag
ridges is applied. Although the ferromagnetic state would be very difficult to
experimentally reach, the logarithmic temperature behaviour of the spin
susceptibility due to the van Hove singularity is found to persist at much
higher temperatures (up to $\sim$97 K).
